Thrombocytopenia in malaria: can platelet counts differentiate malaria from other infections?

OBJECTIVE To determine the accuracy of thrombocytopenia as a diagnostic marker for malaria. STUDY DESIGN Cross-sectional study. PLACE AND DURATION OF STUDY Department of Medicine, 1 Mountain Medical Battalion (Bagh, Azad Kashmir) from July to September 2013. METHODOLOGY Adult patients presenting with a short history of fever without any localizing symptoms or signs were included. Exclusion criteria included patients with fever of > 7 days duration, those in whom an underlying diagnosis could be easily confirmed on the basis of history and physical examination, those on antibiotics/ antimalarials or antiplatelet agents and patients with Dengue fever. Platelet counts in venous whole blood samples were analysed with Sysmex KX-21 Haematology analyzer. Thick and thin peripheral blood smears were then prepared and examined for malarial parasites. Diagnosis of malaria was established on the basis of smear findings. RESULTS There were 245 patients in total. Out of the 109 patients with thrombocytopenia, 61 had vivax malaria. Platelets count was normal in 136 patients, including 4 with vivax malaria. Falciparum malaria was not seen in any patient. All cases with malaria were uncomplicated. Various measures of accuracy thus calculated were sensitivity 93.85%, specificity 73.33%, positive predictive value 55.96%, negative predictive value 97.06%, positive likelihood ratio of 3.52, negative likelihood ratio of 0.08, diagnostic odds ratio 41.94 and diagnostic accuracy of 78.78%. CONCLUSION Thrombocytopenia has an excellent sensitivity and a very good specificity for vivax malaria. Normal platelet counts provide very strong evidence against malaria as the etiology of fever without a focus.