Field evaluation of the QBC technique for rapid diagnosis of vivax malaria.

The QBC (quantitative buffy coat) technique was compared with that of the Giemsa-stained thick blood film (GTF) under field conditions in Junlian and Mingshan counties, Sichuan, China, for rapid diagnosis of vivax malaria. Blood samples were collected from 364 volunteer villagers, and each sample was examined with both the QBC and GTF techniques. For each GTF sample (10 microliters of blood), as many as 300 oil-immersion fields were examined; each QBC tube was inspected for up to 5 minutes. The GTF technique resulted in 86 positive blood samples and 278 negative; the QBC technique indicated 89 positive and 275 negative samples. Relative to the results obtained with GTF, the QBC technique had a sensitivity and specificity of 87.2% and 95.0%, respectively; concordance between the tests was 93.1%. The median time-to-positive diagnosis with the QBC technique (1.12 min) was 11% of that with GTF. The distribution of different developmental stages of Plasmodium vivax parasites was also examined in the centrifuged QBC tubes: all stages except schizonts could be found in the lower part of the platelet zone (the interphase between the monocyte and platelet layers), especially ring forms.