Figure. Probit analysis of laboratories with a positive result (y axes) for chikungunya virus in relation to viral RNA concentration in positive samples (x axes). A) Laboratories using in-house reverse transcription–PCRs (RT-PCRs) (n = 18) had a 50% certainty of having a positive result at 10,000 RNA copies/mL (95% confidence interval [CI] 3,162–19,952). B) Laboratories using a preformulated RT-PCR (n = 13) had a 50% certainty of having a positive result at 1,288 RNA copies/mL (95% CI 416–2,344). Data points represent individual samples in the test panel. Thick line is the regression line calculated on the basis of a probit model (dose-response curve), and thin lines are 95% CIs. Data fit into the model with p<0.00001.