Efficacy of a new rapid diagnostic test kit to diagnose Sri Lankan cutaneous leishmaniasis caused by Leishmania donovani

Abstract

Background: Cutaneous leishmaniasis (CL) in Sri Lanka is caused by Leishmania donovani. This study assessed the diagnostic value of a new rapid diagnostic immunochromatographic strip (CL-Detect™ IC-RDT), that captures the peroxidoxin antigen of Leishmania amastigotes.

Methodology/principal findings: We sampled 74 clinically suspected CL lesions, of which 59 (79.7%) were positive by PCR, 43 (58.1%) by Giemsa stained slit skin smear (SSS) and 21 (28.4%) by the new IC-RDT. All samples which were positive either by SSS or IC-RDT or both were positive by PCR. The sensitivities of the IC-RDT and SSS compared to PCR were 36% and 73%, respectively. Fifteen patients from this endemic region were negative by all three tests. Twenty two clinically non-CL skin lesions from a CL non-endemic region were also negative by all three methods. Specificity and PPV of both IC-RDT and SSS compared to PCR were 100%; the NPVs of IC-RDT and SSS were 37% and 58%, respectively. The median parasite grading of the 59 PCR positive samples was 2+ (1-10 parasites/100 HPFs) and IC-RDT positive lesions was 3+ (1-10 parasites /10HPFs). The duration of the lesion was not associated with IC-RDT positivity.

Conclusions/significance: The median parasite grade of Sri Lankan CL lesions is low. The low sensitivities of SSS and CL Detect™ IC-RDT may be due to low parasite counts or low expression of peroxidoxin antigen in amastigotes of the Sri Lankan L. donovani strain. Our results indicate that negative SSS has to be combined with PCR for confirmation of CL in Sri Lanka. The current commercially available IC-RDT is not suitable to diagnose CL in Sri Lanka; an IC-RDT with improved sensitivity to detect L. donovani would be a valuable addition in the diagnostic tool kit for Sri Lanka.

Fig 1. Venn diagram showing the distribution of positive laboratory results.

Samples were taken from a total of 74 suspected skin lesions. Out of them only 59 became positive by at least one laboratory test. All three tests were positive only in 21 patients.

Fig 2. Comparison of laboratory results.

(2A) ITS1 PCR result of n = 6/59 samples. Lane 1:100bp DNA ladder, Lane 2: negative control, Lane 3: sample 67, Lane 4: sample 65, Lane 5: sample 63, Lane 6: sample 64, Lane 7: sample 74, Lane 8: sample 73, Lane 9: positive control. (2B) Slit skin smear image. Characteristic amastigotes (sample 64; *grade 4+) x1000 magnification. (2C) Tested IC-RDT strips. Left to right; Strip 1: sample 30 (*grade 2+), Strip 2: sample 9 (*grade 4+), Strip 3 = sample 46 (*grade 3+), Strip 4 = sample 64 (*grade 4+), Strip 5 = sample 67 (*grade 2+), Strip 6 = sample 74 (*grade 2+), Strip 7 = sample 63 (*grade 2+), Strip 8 = sample 65 (*grade 3+), Strip 9 = sample 69 (*grade 3+). *grade = parasite grade [33].