Sensitivity of skin snips in the diagnosis of onchocerciasis

To find the optimum number and combination of skin snips for the diagnosis of onchocerciasis, data of six skin snips (two shoulder, two hip, two calf) taken from 776 Liberians aged 12 to 60 years were examined. From these data, proportions of positive skin snips from the hip, or hip and calf areas, for different levels of skin microfilaria density were calculated. These proportions were used in a mathematical model that examined 20 hypothetical populations with different levels of endemicity and skin microfilaria densities. The sensitivity of using two or four snips was then estimated. If only two snips were taken, two hip snips had the highest sensitivity (ranging from 80.7% to 100% according to the population). When the microfilaria density was high, the addition of two extra snips from the calf did not significantly improve the sensitivity. However, when the microfilaria density was low, the combination of two hip and two calf snips substantially improved the sensitivity (ranging from 80.7% to 94%). If fewer than six skin snips are desired for a particular field study, the choice of how many skin snips to be taken should be based on the expected precision required for that study. This in turn can be determined by skin microfilaria density. If such information is lacking, an estimate can be made from the findings of six snips from a small sample.