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Revista da Sociedade Brasileira de Medicina Tropical

Print version ISSN 0037-8682

Rev. Soc. Bras. Med. Trop. vol.47 no.3 Uberaba May/June 2014

https://doi.org/10.1590/0037-8682-0078-2014 

Major Articles

Platelet profile is associated with clinical complications in patients with vivax and falciparum malaria in Colombia

Edgar Leonardo Martínez-Salazar[1]  [2] 

Alberto Tobón-Castaño[1]  [2] 

[1]Grupo Malaria, Universidad de Antioquia, Medellín, Colombia

[2]Facultad de Medicina, Universidad de Antioquia, Medellín, Colombia


ABSTRACT

Introduction

Thrombocytopenia is a common complication in malaria patients. The relationship between abnormal platelet profile and clinical status in malaria patients is unclear. In low and unstable endemic regions where vivax malaria predominates, the hematologic profiles of malaria patients and their clinical utility are poorly understood. The aim of this study was to characterize the thrombograms of malaria patients from Colombia, where Plasmodium vivax infection is common, and to explore the relationship between thrombograms and clinical status.

Methods

Eight hundred sixty-two malaria patients were enrolled, including 533 (61.8%) patients infected with Plasmodium falciparum, 311 (36.1%) patients infected with Plasmodium vivax and 18 (2.1%) patients with mixed infections.

Results

The most frequently observed changes were low platelet count (PC) and high platelet distribution width (PDW), which were observed in 65% of patients; thrombocytopenia with <50,000 platelets/µL was identified in 11% of patients. Patients with complications had lower PC and plateletcrit (PT) and higher PDW values. A higher risk of thrombocytopenia was identified in patients with severe anemia, neurologic complications, pulmonary complications, liver dysfunction, renal impairment and severe hypoglycemia. The presence of thrombocytopenia (<150,000 platelets/µL) was associated with a higher probability of liver dysfunction.

Conclusions

Young age, longer duration of illness and higher parasitemia are associated with severe thrombocytopenia. Our study showed that thrombocytopenia is related to malaria complications, especially liver dysfunction. High PDW in patients with severe malaria may explain the mechanisms of thrombocytopenia that is common in this group of patients.

Key words: Thrombogram; Thrombocytopenia; Platelet; Plasmodium ; Severe malaria; Colombia

INTRODUCTION

Malaria is a disease caused by infection with Plasmodium parasites1 and is a major health problem worldwide. According to the World Malaria Report, 219 million malaria cases and 660,000 malaria-related deaths occurred worldwide in 20102.

Plasmodium falciparum is the main cause of malaria-related death worldwide; however, other species can also cause serious illness3,4. Clinical complications in malaria patients include cerebral malaria, severe anemia (SA), acute kidney failure (AKF), pulmonary edema (PE), severe hypoglycemia, shock, disseminated intravascular coagulation (DIC), acidosis and massive hemolysis. A patient who presents with one or more of these conditions is diagnosed with severe malaria (SM) and has an increased risk of mortality5.

Different organs can be affected during a malaria episode, which results in localized or systemic injury. Hematological changes, especially anemia and thrombocytopenia, are common6,7. Thrombocytopenia has been associated with Plasmodium knowlesi8, Plasmodium falciparum9, Plasmodium vivax, Plasmodium ovale10 and Plasmodium malariae infections11. The evidence is mixed regarding the existence of a correlation between platelet count (PC) and parasitemia810,1214. In a systematic review, Lacerda et al. found no differences in the frequency of thrombocytopenia across species of Plasmodium; the frequency of thrombocytopenia ranged from 24-94% in all studies6. The frequency of thrombocytopenia ranged from 8.0-64.2% in Colombian patients1517.

Various mechanisms have been proposed to explain thrombocytopenia during malaria episodes, including platelet destruction by immune mechanisms14,1820; low medullar platelet production14,21,22; low thrombopoietin (TPO) synthesis23; platelet sequestration in organs, such as the spleen24,25 or brain2628; and systemic sequestration2931. These changes are transient, and in general, patients recover completely after malaria treatment32.

The 2013 World Health Organization (WHO) criteria that define complicated malaria do not include severe thrombocytopenia33. Studies have postulated that malaria should be suspected in travelers who present with fever and thrombocytopenia34. In malaria patients, thrombocytopenia is generally mild or moderate13,17, and bleeding is rare35,36. One study found that severe thrombocytopenia in malaria was an independent predictor of death (odds ratio (OR)=13.3, 95%CI=3.2-55.1)37, but this finding has been contested9. Thrombocytopenia is more frequent in pregnant women with malaria38 and is especially pronounced in primigravidae. Additionally, there is an inverse relationship between parasite density and PC39.

The relationship between thrombocytopenia and clinical presentation is not sufficiently understood, and platelet changes beyond the PC have been either entirely unstudied or not studied in relation to other aspects of the clinical presentation. Moreover, in low and unstable endemic regions such as Colombia, the hematologic profiles of malaria patients and their clinical utility remain poorly understood. The aim of this descriptive study was to characterize the thrombograms of malaria patients from different regions of Colombia and to explore the relationship between clinical status and platelet alterations in ambulatory and hospitalized patients.

METHODS

Patients and sociodemographic characteristics

A retrospective analysis was conducted using clinical and laboratory data obtained from patients from malaria-endemic regions in Colombia. Patients were treated on an outpatient basis for P. falciparum and P. vivax and were enrolled in clinical and epidemiological studies conducted by the Grupo Malaria (Universidad de Antioquia) between 1997 and 20074042. A second study group consisted of malaria patients hospitalized between 2005 and 2010 (A. Tobón: unpublished data). The reference population consisted of patients from all age groups who presented at first- or second-tier health-care facilities in the municipalities of Turbo and Necoclí (Urabá Region, Antioquia State), El Bagre (Bajo Cauca Region) and Tumaco, Guapi and Timbiquí (Pacific coast), as well as individuals who were treated on an inpatient basis at third-tier hospitals in Medellin with malaria diagnosed microscopically by thick smear (Table 1). For all patients with a complete blood count (CBC), a convenience sample was taken. Eight hundred sixty-two patients were sampled.

TABLE 1 - Platelet variables: descriptive statistics and categorical classification. 

PCa PTb MPVc PDWd
Number 862 720 720 654
Mean 134,905 0.112 8.3 17.7
Median 121,000 0.102 8.4 17.9
SD 82,249 0.065 1.93 2.2
Minimum 6,000 0.007 2.6 0.6
Maximum 727,000 0.662 14.6 37.9
Classification by category* n % n % n % n %
 low 564 65.4 258 35.8 120 16.7 23 3.5
 normal 293 34.0 450 62.5 598 83.1 207 31.7
 high 5 0.6 12 1.7 2 0.3 424 64.8

PC: platelet count; PT: plateletcrit; MPV: mean platelet volume; PDW: platelet distribution width; SD: standard deviation

*Reference values:

aPC (platelets/µL): low <150,000; normal 150,000-450,000; high >450-000.

bPT (%): low <0.085; normal 0.085-0.287; high >0.287;

cMPV (fL): low <6.5b; normal 6.5-13.5; high >13.5;

dPDW: low <15.4; normal: 15.4-16.8; high >16.8.

Clinical and laboratory procedures

The thick smear was performed according to the WHO recommendations43. Following a medical assessment, a sample of blood was collected in ethylenediaminetetraacetic acid (EDTA) for cellular and biochemical studies, which included a CBC within 2 hours after sampling. The analysis was conducted in third- and fourth-generation automated analyzers (Cell-Dyn 3200, from Abbott Diagnostics®, Canada; Coulter HmX from Beckman Coulter®, United States of America; Celltac F MEK 8222, from Nihon Kohden®, United States of America and Sysmex KX-21N®, from Sysmex America, Inc. United States of America). Reference values for the platelet variables for the Colombian population were as follows: PC 150-400 x 103 platelet/µL, mean platelet volume (MPV) 6.5-13.5 fl, platelet distribution width (PDW) 15.4-16.8% and plateletcrit (PT) 0.085-0.287%44.

The complications were classified according to the major criteria of the WHO5, along with the additional minor criteria proposed for Colombian endemic regions42. The complications were classified as follows: 1) Liver failure: severe (total bilirubin >3 mg/dl; AST >120IU/L) or mild (total bilirubin >1.5-3mg/dl; AST >80-120IU/L); 2) Thrombocytopenia: profound (<25,000 platelets/µl) or severe (25-50,000 platelets/µl); 3) Renal impairment: severe (blood urea nitrogen (BUN) >60mg/dl or creatinine >3mg/dl) or mild (BUN 41-60 mg/dl or creatinine 1,5-3mg/dl). 4) Anemia: severe (hemoglobin <5g/dl) or moderate (hemoglobin 5-6.9g/dl); 5) Neurologic complication: cerebral malaria (seizures/coma) or extreme weakness; 6) Pulmonary injury: acute respiratory distress syndrome (ARDS)/pulmonary edema or pleural effusion; 7) Acid-base disturbance: severe acidosis (pH <7.35 and HCO3 <15mEq/L) or acidosis (pH<7.35 and HCO3 15-18mEq/L); and 8) Hypoglycemia: severe (<40mg/dl) or moderate (40-49mg/dl).

Statistical analysis

The Kolmogorov-Smirnov test was used to test quantitative variables for normality; the median values were compared using the Mann-Whitney U test when the variables were not normally distributed. Associations between the qualitative variables were analyzed with the Chi-square test. Spearman's correlation coefficient was applied to analyze specific relationships. The level of statistical significance adopted was 5% (p<0.05). All analyses were conducted using the IBM® SPSS® statistics program, version 19 (licensed for Universidad de Antioquia).

Ethical considerations

All patients provided their informed consent to enroll in this study as required by the ethics committee of the Medical Research Institute of the Medical School of the Universidad de Antioquia (Acta 31, July 2002 and Acta 05, June 2005). The review of patient medical records was approved by the hospital ethics committees from Pablo Tobón Uribe Hospital and San Vicente Foundation Hospital (Letter 5282, Oct. 2009).

RESULTS

We reviewed the clinical records of 862 patients with malaria, including 144 (16.7%) inpatients at the San Vicente Foundation Hospital and Pablo Tobón Uribe Hospital in Medellin. All patients were classified in 4 age groups: less than 1 year (n=8, 0.9%), 1-5 years (n=49, 5.7%), 5.1-15 years (n=187, 21.7%) and >15 years (n=618; 77.7%); the precise age was known in 610 patients (mean 25.9 years; standard deviation (SD) 16.6; median 22.8). In total, 62.9% of the patients were male.

In 592 patients, the mean duration of illness prior to seeking treatment was 6.2 days (SD 6.4; median 5.0; range 1-99). The Plasmodium species diagnosed were P. falciparum, 533 (61.8%) patients; P. vivax, 311 (36.1%) patients; and mixed infection, 18 (2.1%) patients. The clinical presentation was classified as acute malaria in 613 (71.1%) patients and complicated malaria in 248 (28.8%) patients.

Platelet profile

The values of PC, PT, MPV and PDW were not normally distributed (Kolmogorov-Smirnov, p=<0.001). Categorical sorting indicated that the most significant alterations were the low PC and high PDW, which were present in approximately 65% of patients (Table 1).

In the 564 (65.4%) patients with PC values below 150,000 platelets/µL, thrombocytopenia was classified in 4 categories (Table 2). Significant thrombocytopenia (<50,000 platelets/µL) was observed in 93 (11%) patients, which included 2% with <25,000 platelets/µL. Other frequent abnormalities included low values for PT and MPV.

TABLE 2 - Thrombocytopenia level and platelet profile by species of Plasmodium

Thrombocytopenia level Platelets/µl P. falciparum P. vivax Mixed infection Total Cumulative %
n % n % n % n %
Profound < 25,000 15 2.2 5 1.3 20 2.3 2.3
Severe 25,000-49,999 40 5.8 29 7.6 4 21.1 73 8.5 10.8
Moderate 50,000-74,999 59 8.6 48 12.6 3 15.8 110 12.8 23.5
Mild 75,000-149,999 241 34.9 116 30.5 4 21.1 361 41.9 65.4
Normal count 150,000-449,000 175 25.4 111 29.2 7 36.8 293 34.0 99.4
High count > 450,000 3 0.4 2 0.5 5 0.6 100.0
Platelet count n=533 n=311 n=18
 mean 136,710 132,002 131,611
 median 119,000 124,000 99,000
 SD 84,909 75,938 107,626
Plateletcrit n=467 n=241 n=12
 mean 0.118 0.102 0.109
 median 0.110a 0.100a 0.129d
 SD 0.068 0.057 0.041
Mean platelet volume n=467 n=241 n=12
 mean 8.5 7.8 8.7
 median 8.5b 8.3b 8.8
 SD 1.7 2.2 1.9
Platelet distribution width n=407 n=237 n=10
 mean 18.2 17.5 17.2
 median 18.5c 17.2c 16.7e
 SD 2.2 2.2 0.8

P.: Plasmodium; SD: standard deviation. Mann-Whitney U test,

ap=0.007;

bp=0.003;

cp=0.000;

dp=0.041;

ep=0.011.

The thrombogram and other variables

We did not identify differences by gender in the median values of PC (p=0.299), PT (p=0.833), MPV (p=0.660) or PDW (p=0.115) or by age (children-adults) in the median values of PC (p=0.914), PT (p=0.972), MPV (p=0.328) or PDW (p=0.640) (Mann-Whitney U test).

The analysis by species is presented in Table 2. The PT, MPV and PDW were significantly lower in the P. vivax cases compared with the P. falciparum cases (Mann-Whitney U test, p<0.05). PC was lower in the P. vivax cases, but this result was not significantly different from that in the P. falciparum cases (p=0.832). The frequency of thrombocytopenia (<150,000 platelets) in the patients with P. falciparum was 66.6% compared with 63.7% in the P. vivax cases. Severe or profound thrombocytopenia (<50,000 platelets) was observed in 55 (10.3%) of the 533 P. falciparum cases, 34 (10.9%) of the 311 P. vivax cases and 4 (22.2%) of the 18 mixed infections (Chi2, p>0.05).

The plateletcrit was higher in mixed infections compared with P. vivax infections (p=0.041), and the PDW was lower in mixed infections compared with P. falciparum infections (p=0.011); additional comparisons by species were not significantly different.

Thrombocytopenia severity and others variables

To explore the relationships between the severity of thrombocytopenia and age, duration of illness and hematological variables, the variables were compared among the patients with no alteration in PC (group A) and the patients with mild (B), severe (C) or profound (D) thrombocytopenia (Table 3). Patient age was not different between the patients with normal or low PCs (p>0.05) but was significantly different between the patients with mild and severe thrombocytopenia (p<0.05), which indicates that younger age could be a risk factor for severe platelet reduction. The median duration of illness was similar between the non-thrombocytopenic patients and the patients with mild changes; however, the duration was significantly higher in the patients with PCs below 50,000/µL (p<0.05). Parasitemia was significantly higher and the erythrocyte count was significantly lower in the patients with PCs below 50,000/µL compared to the other patients groups (Table 3).

TABLE 3 - Thrombocytopenia level stratified by quantitative variables (median values). 

Group A B C D C+Db A vs B A vs C+D
Thrombocytopenia (platelet count) No (>150, 000) Mild-moderate (50,000-150,000) Severe (25,000-49,999) Profound (<25,000) Severe or profound (<50,000) p= (Mann-Whitney U test)
Age 23.3 23.0 19.0 17.0 19.0 0.112 0.129
Duration of disease (days) 4.0 4.0 7.0 7.0 7.0 0.683 0.009
Parasitemia (parasites/µL) 3,720 5,000 8,960 30,556 10,800 0.115 0.000
Hemoglobin (g/dL) 12.2 11.9 11.4 11.5 11.4 0.155 0.029
Erythrocyte count (#/µL) 4.46 4.4 4.2 4.2 4.2 0.461 0.002
Leucocytes (#/µL) 7,000 5,700 5,800 5,450 5,700 0.217 0.000
Lymphocytes (#/µL) 1,955 1,421 1,425 1,073 1,372 0.187 0.000
Monocytes (#/µL) 400 360 318 216 273 0.086 0.012
Neutrophils (#/µL) 4,379 3,732 3,949 3,397 3,700 0.483 0.001
PTia 12.1 11.9 14.1 13.9 14.1 0.195 0.001
PTTa 33.0 33.0 35.5 39.0 36.7 0.871 0.002

PTi: Prothrombin time; PTT: Partial thromboplastin time.

aPTi and PTT in seconds;

bMann-Whitney U test: between A and D, p<0.05 for all comparisons.

The comparison between the patients with normal PC and the patients with any degree of thrombocytopenia showed significant differences in the leukocyte count, with the latter group characterized by significantly lower counts of lymphocytes (p<0.001), monocytes (p=0.003), neutrophils (p=0.010) and eosinophils (p=0.029).

In the cases of severe or profound thrombocytopenia, all variables with the exception of age were significantly different compared with the patients with normal counts. Lower leukocyte and erythrocyte counts, higher coagulation times [prothrombin time (PTi) and partial thromboplastin time (PTT)] and higher parasitemia were observed in the patients with severe or profound thrombocytopenia.

Finally, the comparison between the patients with mild, severe and profound thrombocytopenia revealed no leukocyte count differences (p>0.05). Young age, a longer duration of illness and higher parasitemia were related to severe thrombocytopenia, a lower erythrocyte count and a longer coagulation time.

Clinical aspects

Clinical complications were diagnosed in 248 (28.8%) patients, and 206 (23.9%) patients exhibited complications other than severe thrombocytopenia. The complications were classified using major, 105 (12.2%) and minor, 143 (16.6%) criteria in accordance with the guidelines established by the WHO5 and the Colombian endemic regions, respectively42. The complications were distributed as follows: 1) Liver failure: mild in 96 (11.1%) patients or severe in 74 (8.6%) patients; 2) Thrombocytopenia: severe in 73 (8.5%) patients or profound in 20 (2.3%) patients; 3) Renal impairment: mild in 21 (2.4%) patients or severe in 10 (1.2%) patients; 4) Anemia: moderate in 22 (2.6%) patients or severe in 4 (0.5%) patients; 5) Neurological complications: extreme weakness in 4 (0.5%) patients or seizures/coma in 11 (1.3%) patients; 6) Pulmonary injury: pleural effusion in 5 (0.6%) patients or ARDS/pulmonary edema in 12 (1.4%) patients; 7) Acid-base disturbances: acidosis in 5 (0.6%) patients or severe acidosis in 4 (0.5%) patients; and 8) Hypoglycemia: moderate in 3 (0.4%) patients or severe hypoglycemia in 1 (0.1%).patient.

Patients with complicated malaria (with or without severe thrombocytopenia) exhibited significantly lower median values for PC and PT and higher median values for PDW (Table 4). Median PC values were significantly lower in the patients with complications other than severe anemia or acidosis. In the patients who experienced complications, the median PT was lower and the median PDW was higher compared with the patients who did not experience complications. Furthermore, the PDW was significantly lower in the patients with renal or neurologic complications. The MPV was similar across patients.

TABLE 4 - Median contrast of platelet features by clinical complications (major or minor). 

Group Platelet count Plateletcrit Mean platelet volume Platelet distribution width
n median p n median p n median p n median p
CM
 yes 248 72,000 0.000 183 0.070 0.000 149 8.6 0.537 154 18.9 0.000
 no 614 137,000 537 0.111 571 8.4 500 17.5
CM except thrombocytopenia
 yes 155 105,000 0.000 110 0.086 0.000 110 8.2 0.466 83 19.1 0.000
 no 614 137,000 537 0.111 537 8.4 500 17.5
Liver complication
 yes 170 90,500 0.000 123 0.073 0.000 123 8.4 0.126 99 19.1 0.289
 no 264 129,000 184 0.100 184 8.0 144 18.7
Renal complication
 yes 31 80,000 0.033 25 0.120 0.850 25 8.8 0.212 22 16.7 0.007
 no 401 114,000 280 0.090 280 8.1 219 18.9
Neurologic complication
 yes 15 41,000 0.000 11 0.13 0.876 11 9.7 0.024 11 16.7 0.003
 no 847 122,000 709 0.10 709 8.4 643 17.7
Pulmonary complication
 yes 17 55,000 0.001 11 0.13 0.649 11 9.3 0.278 10 16.7 0.525
 no 845 122,000 709 0.10 709 8.4 644 17.7
Hypoglycemia (<49g/dl)
 yes 4 40,500 0.004 3 0.04 0.010 3 9.8 0.649 2 18.3 0.751
 no 333 119,000 242 0.09 242 8.0 182 19.2
Severe anemia
 yes 26 105,500 0.307 20 0.11 0.956 20 8.8 0.776 18 17.6 0.336
 no 836 121,500 700 0.10 700 8.4 636 17.7
Acidosis (severe or moderate)
 yes 11 32,000 0.276 11 0.13 0.134 11 9.8 0.119 10 16.7 0.392
 no 18 56,500 13 0.13 13 8.3 14 16.7

CM: complicated malaria.

Severe and profound thrombocytopenia (<50,000 platelets/µL) were related to clinical complications (O.R. 1.6; 95% 95%CI=1.5-1.8; p≤0.001). A higher probability of thrombocytopenia was observed in patients with severe anemia (OR=3.2; 95% CI=1.3-7.9; p=0.007), neurologic complications (OR=12.0; 95%CI=4.0-35.0; p≤0.001), pulmonary complications (OR=8.0; 95%CI=3.0-21.0; p≤0.001), liver dysfunction (OR=3.9; 95%CI=2.2-6.9; p≤0.001), renal impairment (OR=3.7; 95%CI=1.7-8.1; p=0.001) and severe hypoglycemia (OR=38.6; 95%CI 3.9-385; Fisher, p=0.002). The presence of thrombocytopenia (<150,000 platelets/µL) was associated with a higher probability of liver dysfunction (OR=2.4; 95%CI=1.5-3.8; p≤0.001).

Finally, we compared the laboratory data among malaria patients with and without severe thrombocytopenia and stratified by Plasmodium specie (Table 5). The laboratory data were significantly different across categories, with the exception of the white blood cell count, serum creatinine and serum glycemia. In the patients infected with P. falciparum, severe thrombocytopenia was related to abnormal values on all liver-function tests, BUN, PTi and PTT; in the patients with P. vivax, a relationship was identified between severe thrombocytopenia and abnormal values on liver-function tests, BUN, serum glucose, erythrocyte and white cell counts.

TABLE 5 - Median values of hematological laboratory tests in relation to severe thrombocytopenia. 

Variable Plasmodium falciparum Plasmodium vivax All patientsa
Severe thrombocytopeniab P valuec Severe thrombocytopeniab P valuec Severe thrombocytopeniab P valuec
n yes no n yes no n yes no
Parasites/µL 391 10,492 4,360 0.000 135 9,760 5,640 0.339 537 10,800 4,680 0.000
Total bilirubin 269 1.8 0.9 0.000 135 1.8 1.1 0.008 416 1.7 1.0 0.000
Direct bilirubin 268 0.9 0.3 0.000 135 0.6 0.3 0.006 416 0.7 0.3 0.000
Aspartate aminotransferase 266 64 33 0.001 100 54 36 0.013 375 57.0 34.0 0.000
Alanine aminotransferase 65 82 27 0.001 100 54 28 0.028 173 54.0 27,0 0.000
Blood urea nitrogen 269 18.0 12.7 0.002 117 16.9 11.1 0.005 379 16.9 12,0 0.000
Prothrombin time 139 14.3 11.8 0.000 15 12.4 14.0 0.111 159 14.1 12,0 0.000
Partial thromboplastin time 142 36.0 32.3 0.005 14 44.0 34.7 0.096 160 36.7 33,0 0.000
Hemoglobin 533 11.4 11.9 0.234 311 11.1 12.2 0.034 844 11.3 12,1 0.029
Erythrocyte count/µL 512 4,185 4,400 0.065 261 4,080 4,500 0,004 787 4,185 4,430 0,002
White blood cells/µL 531 6,100 6,000 0.784 310 4,850 6,500 0.001 860 5,700 6,000 0.054
Creatinine 278 1.0 1.0 0.599 138 0.90 0.94 0.528 412 1.0 0.9 0.425
Serum glycemia 230 94.5 100 0.261 103 183 109 0.028 337 100 103 0.454

aIncluding mixed infections;

b<50,000 platelets/µL;

cMann-Whitney U test.

DISCUSSION

In addition to anemia, a reduction in the number of platelets is one of the more well-known hematologic changes observed in patients with malaria. While platelet-count changes are particularly associated with P. falciparum malaria9,12,45, this pathology has also been identified in malaria caused by other species, such as P. vivax36,46. Changes in other platelet parameters, such as PT, MPV or PDW, have not been well described in malaria patients.

Some studies have found that patients infected with P. vivax or P. falciparum develop anemia, monocytosis, eosinopenia and lymphopenia45. This study found a progressive reduction in hemoglobin levels and white blood cell counts with reduced PCs, which resulted in anemia, monocytopenia, neutropenia and lymphopenia. Thrombocytopenia presented frequently in the patients infected with P. falciparum (67%) and P. vivax (64%). The PC did not differ by species, which suggests that a similar mechanism governs this interaction in P. vivax and P. falciparum infections. Significant differences in PT and MPV were observed among species, but the values remained within the normal range. The PDW was significantly higher in the patients infected with P. falciparum compared with those infected with P. vivax; however, both median values were above the normal range. These findings and the results of various studies that have demonstrated a rapid recovery of PCs following the initiation of antimalarial treatment suggest that thrombocytopenia is related to a systemic endothelial and platelet activation rather than medullary suppression. Systemic endothelial activation likely mediates platelet sequestration and does not depend exclusively on the formation of platelet aggregates and parasitized erythrocytes, as observed in P. falciparum infections47.

Interestingly, we found that the patients with P. vivax infection and severe thrombocytopenia had more significant changes in leukocyte and erythrocyte counts compared with the patients without thrombocytopenia. These changes, which were not identified in the patients infected with P. falciparum, favor a stronger inflammatory response in severe P. vivax infections that can affect bone marrow via the production of tumor necrosis factor alpha (TNF-α). This hypothesis is supported by a study that identified increased production of TNF-α in P. vivax malaria and a stronger host immune response in P. vivax malaria compared with P. falciparum malaria48.

Various mechanisms have been proposed to explain thrombocytopenia in malaria; however, its clinical significance remains uncertain. Thrombocytopenia can be interpreted as evidence of damage and has been associated with clinical complications and high parasitemia9,12,37. However, some studies have suggested that thrombocytopenia has a protective effect, which reduces the probability of red blood cell aggregation28 or mediates parasite destruction. The latter effect has been observed in vitro in the early stages of erythrocytic infection with P. falciparum and in a mouse model with Plasmodium chabaudi49. In this study, we identified a significant relationship between severe thrombocytopenia and other clinical complications, but we were not able to establish causality.

Thrombocytopenia has been associated with disturbances in coagulation and has been implicated in cases of disseminated intravascular coagulation (DIC)50. In children with severe malaria, prolonged PTi, PTT and thrombin time (in 47.5, 35.0 and 62.5% of patients, respectively) may lead to platelet hypoaggregation46. Activation of coagulation may also be responsible, in part, for thrombocytopenia6. Our study showed that the patients infected with P. falciparum, but not P. vivax, had more prolonged PTi and PTT when the thrombocytopenia was more severe. This difference may be the result of increased procoagulant activity in P. vivax compared with P. falciparum infections, which develops in response to increased endothelial activity induced by TNF-α levels and the consequent formation of thrombin-antithrombin III48.

In this study, patients with severe or profound thrombocytopenia showed higher parasitemia compared with patients without thrombocytopenia, which is in agreement with previous findings. Low PCs have been associated with increased parasite load. This association may result from sensitization induced by the parasitized red blood cells in the platelet, with consequent increased platelet sensitivity to adenosine diphosphate (ADP) and higher dense-granule secretion45,46. These changes could ultimately promote platelet aggregation on the endothelium, which has been demonstrated in cerebral malaria27.

Plateletcrit is a measure of platelet mass, the clinical significance of which should be interpreted in terms of the number and size of the platelets. Whitfield et al.51 demonstrated that in healthy subjects, there was an inverse relationship between the platelet size (MPV) and the platelet number (PC) and proposed that the total platelet mass is closely regulated by changes on MPV or PC. We found that the PT was low in 36% of patients with malaria. This finding may be explained by a combination of low PC with normal MPV (63%), low PC and MPV (35%) and normal PC with low MPV (2%). The reduction in circulating platelet mass is best explained by a reduction in platelet number rather than a reduction in platelet size. It is suspected that in malaria, sequestration leads to pseudothrombocytopenia52.

Some studies have identified a relationship between the decrease in PC and the appearance of clinical complications, such as anemia, respiratory distress syndrome and cerebral malaria37. We found that the patients with severe (25,000-49,999 platelets/µL) or profound thrombocytopenia (<25,000 platelets/µL) had more pathological changes in their clinical status compared with the patients without thrombocytopenia. Furthermore, parasitemia, the duration of disease, and TP and TPP increased with an increasing severity of thrombocytopenia (mild→moderate→severe→profound), whereas erythrocyte and leukocyte counts decreased.

In this study, the patients who experienced complications had lower PC and PT values and higher levels of PDW compared with the patients who did not experience such complications. Specifically, the median PCs were significantly lower in the patients with hepatic complications (90,500 vs. 129,000; p≤0.001), renal complications (80,000 vs. 114,000; p=0.017), neurologic complications (42,000 vs. 122,000; p≤0.001), pulmonary complications (55,000 vs. 122,000; p=0.001) and severe hypoglycemia (40,500 vs. 119,000; p=0.004). The low PC may result from the adhesion of platelets to the sites of vascular injuries and the mediation of malaria-parasitized red blood cells (pRBC) sequestration to the endothelium through von Willebrand-factor strings53.

When platelets decrease in number, bone marrow megakaryocytes are stimulated by thrombopoietin, and their nucleus becomes hyperlobulated with a higher deoxyribonucleic acid (DNA) content. These stimulated megakaryocytes produce larger platelets. Thus, platelets with a higher MPV are expected to be present in autoimmune thrombocytopenia when megakaryocytic stimulation is present. Conversely, platelets with a lower MPV are expected in thrombocytopenic states associated with marrow hypoplasia or aplasia54. Interestingly, 83% of the patients studied had normal MPV, and it appears that neither of these two mechanisms associated with thrombocytopenia was significantly activated.

The examination of PDW with MPV provides information regarding thrombocyte volume disturbance. Thrombocyte volume heterogeneity occurs because of the production factors in the bone marrow. PDW is an index of thrombocyte volume heterogeneity similar to erythrocyte distribution, and both MPV and PDW are markers of platelet immaturity and platelet activation55. A high PDW predominated in the patients studied (65%), and we identified significantly higher PDW in the total sample of severe patients; however, the analysis for each complication indicated conflicting results, which could be a result of the differential patterns of inflammation in the spectrum of clinical complications or statistical effects based on the low number of cases when the complications were individually analyzed.

In healthy human subjects, no correlation was identified between PCT, MPV or PDW values and platelet aggregation results54; however, these relationships could be modified in the presence of inflammation.

It has been reported that PDW increases over storage time because of the formation of abnormally small and large platelets54, and it is well known that pseudo-thrombocytopenia occurs as a result of exposure to EDTA when used in blood sampling56. There is a need for multi-center and prospective studies with large sample sizes to elucidate the utility and clinical importance of these parameters in malaria.

Our results indicate that thrombocytopenia accompanies malaria complications, especially liver dysfunction. Major changes in total bilirubin, direct bilirubin, aspartate aminotransferase, alanine aminotransferase, prothrombin time and partial thromboplastin time were observed in severe thrombocytopenia (<50,000 platelets/µL). The PCs were significantly lower in the patients who experienced liver, renal, neurologic, and pulmonary complications and severe hypoglycemia.

The liver is a key regulator of platelet production through the synthesis of TPO. Thus, hepatic dysfunction may explain, in part, the thrombocytopenia observed in these patients; however, more data are required because TPO appears to rise during infection by P. falciparum, even when liver function is compromised23.

Differences between P. falciparum and P. vivax infections may arise from the different mechanisms by which the physiopathology of thrombocytopenia is mediated, i.e., via clumping in P. falciparum and via medullar suppression in P. vivax. A better understanding of these mechanisms is still needed.

Despite the inherent limitations of retrospective analyses, these results may contribute to the understanding of severe thrombocytopenia as a negative prognostic marker in patients with malaria. Our findings suggest a need to study hepatic function and to establish close clinical monitoring of malaria patients. Furthermore, there is a substantial need for prospective studies to elucidate the utility and clinical importance of these parameters in malaria.

ACKNOWLEDGMENTS

Our sincere thanks to Grupo Malaria, Universidad de Antioquia, who provided partial data analyzed in this study, and to Felipe Miranda, M.D., Ana del Mar Cortina, M.D., Ana Maria Cadavid, Lia Palacio and Shirley Jolianiz, who are medical students at the School of Medicine at the University of Antioquia, for their help in acquiring hospital medical records. This project was funded by the Universidad de Antioquia, Colombia (Grupo Malaria de la Facultad de Medicina, Programa Jóvenes Investigadores de Vicerrectoría de Investigación y CODI- Estrategia de Sostenibilidad 2014-2015).

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Received: April 14, 2014; Accepted: June 18, 2014

Address to: Dr. Edgar Leonardo Martínez Salazar. Grupo Malaria/Facultad de Medicina/Universidad de Antioquia. Sede de Investigación Universitaria. Calle 62 #52-59, Laboratorio 610, 050001 Medellín, Colombia. Phone: 57 4 219-6487. e-mail: elmsmd@gmail.com

CONFLICT OF INTEREST The authors declare that there is no conflict of interest.

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