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Prevalence of nodules
Evaluating the
diagnostic value
of nodule palpation requires a reference diagnosis
which serves as gold standard. For the data of the graph below,
we have used skin snip diagnosis as the reference.
The proportions of true-positive, false-positive, false-negative and
true-negative diagnoses depend on the age of the patients.
Two main findings are:
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the probability of a true-positive palpation increases strongly with age
(simply because the number of nodules increases with age and thus,
the probability to find at least one of them increases).
-
the probability of a true-negative palpation decreases strongly with age
(simply because the number of nodules increases with age and thus,
the probability to find none of them decreases).
These two examples show a major obstacle in the diagnosis of parasitic infections:
the diagnosis result for a patient depends on how many parasites he/she harbours!
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Figure: Age-dependent proportions of 2560 individuals with
true-positive (PTP),
false-positive (PFP),
false-negative (PFN) and
true-negative (PTN) palpation diagnosis,
taking skin snip diagnosis as reference. The proportions are classified
such that sample sizes across age classes are comparable.
(Source: Duerr et al., 2007)
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Related pages:
Nodules,
Diagnosis by palpation of nodules.
Further reading:
Duerr HP, Raddatz G, Eichner M, 2007. Diagnostic value of nodule palpation in onchocerciasis. Transactions of the Royal Society of Tropical Medicine and Hygiene: in press.
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