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Diagnostic value of nodule palpation
The low sensitivity of nodule palpation and strong age-dependencies in the sensitivity
and the specificity makes this kind of diagnosis to an unreliable instrument.
The diagnosis becomes even poorer in regions with a low prevalence of infection
which decreases the positive predictive value of nodule palpation.
The graph below shows for various specificities how the prevalence determined by palpation
depends on the ('true') prevalence determined by skin snip diagnosis,
together with data of 14 West African villages.
The prediction strongly depends on the specificity;
e.g. a palpated prevalence of 20% could imply a 'true' prevalence between 0% and 45%,
for specificities ranging between 80% and 100%.
If nodule palpation were 100% specific, the prevalence would always
be underestimated because sensitivity is not perfect
(false-negative diagnoses decrease the palpated prevalence).
With decreasing specificity, the predictive value of nodule palpation drastically diminishes;
e.g. for a specificity of 60%, a palpated prevalence of 40% predicts
the 'true' prevalence to range roughly between 0 and 50%.
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Figure: Predicted prevalence (Vt) as a function of the
palpated prevalence (Vo),
i.e. proportion of positive palpation diagnoses under the assumption
that 30% of the nodules are palpable.
Different specificities are indicated by C=
.
The dashed line represents perfect agreement.
Dots refer to prevalences found in 14 West African villages.
At low prevalence, palpation frequently overestimates the true prevalence
because of false-positive diagnoses. Predictions about the true prevalence
become increasingly unreliable as specificity decreases. (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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