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

Background: The standard method to diagnose Loa loa infection and quantify microfilarial density (MFD) is the microscopic examination of calibrated thick blood smears (TBSs). In 1950, it was noticed that successive L. loa MFD samples from a single capillary puncture could exhibit up to 20% variation. Although loiasis treatment allocation is based on MFD to prevent serious adverse events (SAEs), data on this variability are scarce. There are also no guidelines supporting the collection and analysis of one or two TBSs.

Methods: We assessed the variability of two successive L. loa MFD samples (MFD and MFD), 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 the impact of MFD variability on MFD thresholds relevant to loiasis treatment protocols.

Results: The MFD was found to have increased in 63% (1145/1826) of TBS pairs and to have decreased in 37% (681/1826) of TBS pairs. The MFD were on average 28% higher than the MFD. These variations drove a total of 333 (17.4%) changes in MFD classes according to loiasis treatment protocol, including 210 (11.3%) class increases. TBSs generated from blood samples from subjects with lower MFD (1-1000 mf/ml) or lower mean arterial pressure (MAP; 55-80 mmHg), or from blood samples collected at an earlier hour time-point (10:00-10:59 a.m.) were more subject to MFD 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 MFD 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 post-treatment SAEs. Further studies are needed to verify this variability in larger samples as well as confirm the potential explanatory variables identified.