Diagnosing visceral leishmaniasis with the recombinant K39 strip test: Experience from the Sudan

We compared a strip test employing recombinant K39 (rK39) antigen and protein A/colloidal gold as read-out agents with the rK39 ELISA for IgM and IgG antibodies and the direct agglutination test (DAT) using 55 sera from patients with parasitologically confirmed visceral leishmaniasis (VL). The rK39 strip test was positive in 37/55 (67%), the DAT in 50/55 (91%) at ≥ 1:1600 cut-off value and in 47/55 (85%) at ≥ 1:6400 cut-off value. The rK39-ELISA gave positive IgG results for all sera; those who had a positive strip test had significantly higher IgG levels than those with a negative strip test (31.1 (SD = 3.6) and 17.7 U/ml (SD = 9.8), respectively, P < 0.0001). A total of 31/55 (56%) sera showed a positive IgM result; of these 27 (49%) had a positive strip test. We tested 115 apparently cured VL patients with the strip test during follow-up; 68 were also tested with DAT. In the strip test, 25-43% of patients had a positive result at time points 3, 6, 9 and 12 months after treatment; for DAT (cut-off ≥ 1:1600) these results were 67-83%. In neither test did a significant decrease in positivity rates occur over time (P = 0.37 for the strip test, P = 0.17 for the DAT). No correlation (P = 0.33) was found between a positive strip test and a positive DAT result (cut-off ≥ 1:1600), indicating that the strip test and DAT are complementary rather than interchangeable. Of 61 endemic controls two (3%) had a positive strip test result; both had a positive leishmanin skin test. The rK39 strip test has the ideal format for use in the field, but its sensitivity is limited; like DAT, but to a lesser extent, it remains positive after treatment.