Low diagnostic performance of thick blood smears of 50 µl in comparison with direct examination of 10 µl blood and the leukoconcentration technique of 5ml blood among loiasis-suspected patients with low microfilaremia in Gabon, Central Africa, using the STARD-BLCM guidelines

Table 2 Prior distributions derived from previous literature for the disease prevalence and performance characteristics of the tests

Models	Prior distribution
	Prevalence	Leukoconcentration				TBS-50/direct blood examination
	Prevalence	Sensitivity	Specificity	PPV	NPV	Sensitivity	Specificity	PPV	NPV
I	Beta(4.61,48.21)	Beta(52.21,0.33)	Beta(202.31,30.25)	Beta(84.49,29.75)	Beta(122.37,0.33)	Beta(15.72,8.02)	Beta(63.65,0.33)	Beta(10.73,0.33)	Beta(80.46,6.36)
II	Beta(1,1)	Beta(1,1)T(1-sp[1])^a	Beta(1,1)	Beta(1,1)	Beta(1,1)	Beta(1,1)T(1-sp[1])	Beta(1,1)	Beta(1,1)	Beta(1,1)
III	Beta(4.61,48.21)	Beta(1,1)T(1-sp[1])	Beta(1,1)	Beta(1,1)	Beta(1,1)	Beta(1,1)T(1-sp[1])	Beta(1,1)	Beta(1,1)	Beta(1,1)

PPV positive predictive value, NPV negative predictive value
^aT(1-sp[1]) = technical term used in JAGS (Just Another Gibbs Sampler) software when defining the formula of the Hui-Walter BLCA model. This addition to the default formula is a safeguard that would have an effect in scenarios when JAGS estimates the Se/Sp to be 0%. This additional “T(1-sp[1])” component of the prior then forces JAGS to consider this Se/Sp as 100% instead, as 0% Se/Sp = 100% Se/Sp after simply switching the labeling of the test results. For the models used in this paper, inclusion/exclusion of “T(1-sp[1])” had no effect on the estimates of the test performance characteristics, meaning that this safeguard remained unused
To calculate priors for the thick smear test, data from Noireau and Apembe [28] were used. Se: 66.7% (49.8–80.9); Sp: 100.0% (97.7–100.0); PPV: 100.0% (86.8–100.0); NPV: 93.0% (87.6–96.0). For the leukoconcentration, data from Bouyou-Akotet et al. [36] were used: Se: 100% (97.2–100.0); Sp: 87.1% (83.2–90.5); PPV: 74.1% (67.0–80.5); NPV: 100.0% (98.8–100.0)

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