Variability of Loa loa microfilarial counts in successive blood smears and its potential implication in drug-related serious adverse events

Abstract

Background: The standard method to diagnose Loa loa infection and quantify microfilarial density (MFD) is the microscopic examination of calibrated thick blood smears (TBS). In 1950, it was noticed that successive L. loa MFDs from a single capillary puncture could exhibit up to 20% variation. Although loiasis treatment allocation is based on MFD to prevent serious adverse events (SAE), data on this variability are scarce. There are also no guidelines supporting the collection and analysis of one or two TBS. Methods: We assessed the variability of two successive L. loa MFDs (MFD₁ and MFD₂), collected from 255 patients. We analyzed the influence of sex, age, weight, heart rate, arterial pressure, body temperature, and sampling time on MFD variability, as well as variability’s impact on MFD thresholds relevant to loiasis treatment protocols. Results: 63% (1145/1826) of TBS pairs exhibited an MFD₂ increase, while only 37% (681/1826) exhibited a decrease. MFD₂ were on average 28% higher than MFD₁. These variations drove a total of 333 (17.4%) MFD class changes according to loiasis treatment protocol, including 210 (11.3%) class increases. TBS sampled from subjects with lower MFD (1-1 000 mf/mL), lower MAP (55-80 mmHg) and sampled at an earlier hour (10:00-10:59 am) were more subject to MFD₂ variability in a multivariate analysis. The MFD relative change was not constant over time for a given person. Conclusions: We observed a trend towards an increase in MFD₂ with an important variability between samples that may impact loiasis treatment allocation. We suggest that systematically sampling at least two successive TBSs might allow better MFD assessments to prevent posttreatment SAEs. Further studies are needed to verify this variability in larger samples as well as confirm the potential explanatory variables identified.