Abstract
BackgroundLymphatic filariasis (LF) infection is generally diagnosed through parasitological identification of microfilariae (mf) in the blood. Although historically the most commonly used technique for counting mf is the thick blood smear based on 20 µl blood (TBS20), various other techniques and blood volumes have been applied. It is therefore a challenge to compare mf prevalence estimates from different LF-survey data. Our objective was to standardise microfilaraemia (mf) prevalence estimates to TBS20 as the reference diagnostic technique.MethodsWe first performed a systematic review to identify studies reporting on comparative mf prevalence data as measured by more than one diagnostic test, including TBS20, on the same study population. Associations between mf prevalences based on different diagnostic techniques were quantified in terms of odds ratios (OR, with TBS20 blood as reference), using a meta-regression model.ResultsWe identified 606 articles matching our search strategy and included 14 in our analyses. The OR of the mf prevalences as measured by the more sensitive counting chamber technique (≥ 50 µl blood) was 2.90 (95% confidence interval (CI): 1.60–5.28). For membrane filtration (1 ml blood) the OR was 2.39 (95% CI: 1.62–3.53), Knott’s technique it was 1.54 (95% CI: 0.72–3.29), and for TBS in ≥ 40 µl blood it was 1.37 (95% CI: 0.81–2.30).ConclusionsWe provided transformation factors to standardise mf prevalence estimates as detected by different diagnostic techniques to mf prevalence estimates as measured by TBS20. This will facilitate the use and comparison of more datasets in meta-analyses and geographic mapping initiatives across countries and over time.
Highlights
Lymphatic filariasis (LF) infection is generally diagnosed through parasitological identification of microfilariae in the blood
To be able to compare our results with earlier approaches for standardising mf prevalences based on scaling factors, we translated the modelpredicted odds ratio (OR) to relative risk (RR) functions; because for a given OR, the RR depends on the prevalence of the reference group, we report derived RRs for different levels of prevalences based on TBS20
Blood sampling was done during day-time after provocation with a single 100 mg dose of diethylcarbamazine citrate (DEC) [15]
Summary
Lymphatic filariasis (LF) infection is generally diagnosed through parasitological identification of microfilariae (mf ) in the blood. Our objective was to standardise microfilaraemia (mf ) prevalence estimates to TBS20 as the reference diagnostic technique. The diagnostic accuracy varies between methods, depending on timing of specimen collection (there is variation in the periodicity of LF parasite species in the blood), the type of blood sample (venous or capillary blood), blood volume examined, and blood sample processing methods (e.g. concentration, filtration, dehaemoglobinisation, etc.). The accuracy of detecting mf is primarily influenced by the quantity of blood sampled [2], with larger blood volumes associated with higher sensitivity and higher mf prevalences. Concentration methods with larger blood volumes may be more sensitive for the detection of especially low mf density infections, but collection of e.g. 1 ml venous blood volumes is not carried out in field conditions. The assessment of geographical differences and trends over time in LF-survey results may be hindered by the use of different diagnostic techniques that each have different diagnostic sensitivities and specificities
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