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Diagnosing onchocerciasis by palpation of nodules
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The parasite Onchocerca volvulus aggregates within subdermal
nodules which can be palpated. Diagnosis by palpating nodules, however,
is uncertain because nodules may be very small, located in deeper
layers of the skin, etc. Diagnostic measures are difficult to derive
and depend on the age of the patients. Is diagnosis by palpation
sufficiently reliable to support epidemiological decision making?
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The sensitivity of nodule palpation depends on the parasite burden,
because a patient with many nodules is more likely to be found "positive"
than a patient with few nodules. How can we determine the sensitivity
of nodule palpation when at the same time exactly this depends on knowing
how many nodules a patient harbours?
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Data on nodule palpation show that the diagnosis strongly depends on the
age of a patient. It is important to look at the proportions of true-positive,
false-positive, false-negative and true-negative diagnoses.
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Compiling the information on sensitivity, specifity, and the true and
the negative predictive value allows for predicting the true prevalence
of onchocerciasis from a prevalence which has been determined by palpation.
However, it turns out that only slight insights can be gained through nodule palpation.
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