If a positive acute-phase IgM result were used to diagnose acute leptospirosis, 27 true infections would be detected (sensitivity 62.8%, 95% CI 46.7—77.0) but 137 patients would be diagnosed erroneously (specificity 77.0%, 95% CI 73.4–80.3, ROC area 0.699 [95% CI 0.62–0.77]). Here, CD40LG is linked to infection.