Field application of Lepto lateral flow for rapid diagnosis of leptospirosis

The Lepto lateral flow assay for leptospirosis was evaluated at a primary health centre in the Andaman Islands, where leptospirosis is endemic. One hundred and seventeen suspected patients were included in the study; acute serum samples were collected from all of them and convalescent samples from 104. The standard criteria for diagnosis of leptospirosis were: (i) isolation of leptospires from blood, (ii) seroconversion in microscopic agglutination test (MAT) with a minimum titre of 100, (iii) a fourfold rise in titre in MAT or (iv) a MAT titre of 400 or more if only a single sample was available. The results of the lateral flow test were compared with these criteria. Lepto lateral flow had sensitivity of 52.9 % (37/70) in the first week of illness and 86 % (49/57) during weeks 2-4. The corresponding specificities were respectively 93.6 % (44/47) and 89.4 % (42/47). The sensitivity was 34.3 % (12/35) on days 2-3 of the illness, 63.3 % (14/22) on days 4-5 and 84.6 % (11/13) at the end of the first week. The test had a positive predictive value of 92.5 % (37/40) during the first week and 90.7 % (49/54) subsequently. Corresponding negative predictive values were respectively 57.1 % (44/77) and 84 % (42/50). Agreement of the results with the standard criteria was low during the first week, but high during weeks 2-4, with a kappa value of 0.7491. The positivity rates of the tests showed a logarithmic relationship with the MAT titres of samples (r(2) = 0.9271). All indices of validity and utility of lateral flow were similar to those of IgM ELISA and Lepto dipstick. The test can be performed at the bedside of the patient, as whole blood can also be used for testing.