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OBJECTIVES To evaluate a modification of the mini anion exchange centrifugation test (mAECT) for the diagnosis of Trypanosoma brucei (T.b.) gambiense human African trypanosomiasis (HAT). To increase its sensitivity, this test uses 350 mu l of buffy coat withdrawn from 5 ml of blood instead of blood. METHODS The new protocol was first tested experimentally on serial dilution of trypanosomes and was then further evaluated under field conditions on 57 patients with HAT diagnosed during a medical survey in Guinea. RESULTS Experimentally, the use of buffy coats improved mAECT sensitivity at least five fold and enabled to consistently detect parasites in blood at a concentration of 10 trypanosomes/ml. During the field evaluation, more patients tested positive by mAECT-bc (96.5%) than by mAECT-blood (78.9%, chi(2) = 6.93, P = 0.008) and lymph juice examination (77.2%, chi(2) = 7.67, P = 0.005). Furthermore, the number of parasites per collectors was significantly higher (7.2 vs. 2.6, P = 0.001) when buffy coats were used instead of blood. CONCLUSION The use of the mAECT-bc protocol enabled a significant improvement of HAT parasitological diagnosis in Guinea, without any additional costs. It would deserve to be tested in other T.b. gambiense endemic areas.