Platelet count and parasite density: independent variable in Plasmodium vivax malaria.

The occurrence of thrombocytopenia in Plasmodium vivax malaria is well-documented. In spite of being a common observation, its prognostic implications in context with parasite density has not been evaluated. In view of the paucity of data, we attempted to study the correlation of platelet count with parasite density in P. vivax monoinfection. This prospective study was carried out on patients attending outpatient clinic from June 2011 to December 2011. The study was conducted on patients coming to out patient department at the time of first consultation for fever in which diagnosis of P. vivax monoinfection was made by peripheral blood smear (PBS) and rapid diagnostic test (RDT). Patients having severe malarial manifestations were enrolled in another observational study (under process) and confirmation of malarial species in them was made by PCR analysis. Due to inclusion of large number of patients in the present study, we used an ‘OptiMAL® Rapid Malaria Dipstick Test’ to rule out concomitant low density P. falciparum mixed infection. This test detects the presence of Plasmodium lactate dehydrogenase (pLDH); an enzyme produced both by the sexual and asexual forms of the parasite. The presence of pLDH is revealed using monoclonal antibodies directed against isoforms of the enzyme. This test detects parasitemia levels of 100–200 parasites per μl of blood (corresponding to a parasitemia of 0.002–0.004%). Another point against the presence of associated P. falciparum infection is that such a low parasitemia of P. falciparum so as to be undetectable both by the dipstick test and standard microscopy, is highly unlikely to cause such profound thrombocytopenia. The possibilities of other concurrent similar illness causing thrombocytopenia were ruled out by appropriate and stringent laboratory investigations. Details are given in our previous study1. Platelet count was done by fully automated counter and the results were derived from directly measured platelet pulses, multiplied by a calibration constant and expressed in 1000 thrombocytes/l of whole blood. A written informed consent was mandatory and those who refused to give the written consent or had other concurrent illness were not included in the study. This study includes 599 patients of P. vivax monoinfection, but parasite density was done only in 546 patients fulfilling the eligibility criteria. The age distribution range included in the study was 1–60 yr and sex ratio was 2.33 (male : female = 382 : 164). The mean ± SD platelet count and mean ± SD parasite density were 90199.6 ± 57414.9 and 6733.4 ± 9523.8, respectively. Thrombocytopenia (<150,000/mm3) was present in 85.71% (468/546) patients with a mean platelet count of 72715.8 ± 38625.4. Statistical analysis done by correlation coefficient did not show any association between platelet count and parasite load [correlation coefficient (R2) = 0.000] (Fig. 1). All these patients were treated as per WHO guidelines.