Diagnostic value of nodule palpation in onchocerciasis

Nodule palpation is the major diagnostic tool for determining the prevalence of infection in areas of the African Programme for Onchocerciasis Control (APOC) and is recommended for identifying communities at risk and selecting them for mass drug administration. The diagnostic value of palpation, however, has not been quantified in terms of sensitivity and predictive values. We derive these measures from the probability that a nodule is palpable, which has been estimated by stochastic simulations from an extensive pre-control database. We show that nodule palpation is only reliable in highly endemic areas and that false-positive diagnoses can lead to considerable misclassifications of regions where endemicity is actually low. Its diagnostic precision is poor because of large intra- and inter-individual variability. The findings underline the need for further development of available diagnostics that allow long-term monitoring when endemicity declines.