Diagnostic accuracy of rapid diagnostic tests for the early detection of leptospirosis
Leptospirosis is often misdiagnosed with several other tropical febrile illnesses in Malaysia due to similarities in clinical manifestations. Although treatment regimens could be started based on clinical judgments, early diagnosis has become paramount as a guide to chemotherapeutic interventions. Confirmed laboratory diagnosis through MAT or PCR is time consuming and usually available only in reference laboratories and not practical in healthcare settings. Rapid and easy to perform diagnostic tests are widely used in these settings as the point of care diagnosis. The present study was undertaken to compare the diagnostic performance of two IgM based immunodiagnostic assay kits for acute leptospirosis.
A total of 50 serum samples were collected from patients clinically suspected for acute leptospirosis on admission in the Hospital Serdang, from June 2016 to June 2017. All the samples were subjected to MAT, lipL32 PCR and the two rapid tests (Leptocheck-WB and ImmuneMed Leptospira IgM Duo Rapid test).
Out of the 50 clinically suspected patients sampled, 19 were confirmed positive for leptospirosis. Six (12%) were confirmed by MAT and 13 (26%) by PCR. Similarly, of the 50 clinically suspected cases, 17 (34%) showed positivity for Leptocheck-WB and 7 (14%) for ImmuneMed Leptospira IgM Duo Rapid test. The overall sensitivity and specificity was 47.37% and 80.65% for Leptocheck-WB, and 21.05% and 90.32% for ImmuneMed Leptospira IgM Duo Rapid test. In another set of previously confirmed MAT positive samples (1:400–1:3600) obtained from a reference laboratory, Leptocheck-WB showed higher sensitivity (90.72%) than ImmuneMed Leptospira IgM Duo Rapid test (40.21%), and comparable specificity for ImmuneMed Leptospira IgM Duo Rapid test (88.89%) and Leptocheck-WB (82.86%).
The sensitivity was higher for Leptocheck-WB and had a comparable specificity with ImmuneMed Leptospira IgM Duo Rapid test. Therefore, based on the present study, Leptocheck-WB is found to be a more sensitive rapid immunodiagnostic test for acute leptospirosis screening in hospital settings.
The neglected tropical illness leptospirosis caused by the spirochete Leptospira, is now an alarming re-emerging zoonosis with a worldwide distribution. In Malaysia, leptospirosis is gazetted as a notifiable disease since December 2010. The number of cases according to the data from the Ministry of Health Malaysia (MOH) showed an increase from 3665 in 2012 to 5284 in 2016. Leptospirosis is a biphasic infection, the first phase (acute or septic phase) commences from 3 to 10 days of disease onset and the second phase (immune phase) ranges from 7 to 14 days .
During the acute phase, the bacteria can be found in the blood and then migrate and reside in the kidney where it continues to be shed in the urine. While in the second phase, a detectable number of antibodies develops and this stage coincides with the disappearance of the bacteria in the blood. Confirmatory laboratory diagnosis for leptospirosis involves testing for antigen (bacteria by culture or PCR of Leptospira pathogenic genes) in the first phase and antibodies in the second phase through the gold standard microscopic agglutination test (MAT).
Culture is not suitable for early diagnosis as Leptospira takes two weeks to four months to grow. On the other hand, PCR based detection, although gives a confirmatory diagnosis, it involves DNA extraction, technical expertise, expensive PCR machines and reagents, which limits the feasibility in many health care facilities. MAT is technically tedious and interpretations are very subjective and most importantly requires a greater panel of live leptospiral cultures to serve as antigens with the regular incorporation of new local and international serovars.
In Malaysia, MAT is available only at the National Leptospirosis Reference Centres such as the Institute for Medical Research located at capital Kuala Lumpur as well as the Zonal Public Health Laboratories. Considering all these shortcomings and challenges, a rapid test is highly desired and mostly preferred in hospitals in Malaysia, as it is a point of care test that can be performed in-house, and it is fast, technically simple and can be easily interpreted. It is pertinent to note that, there are several rapid tests commercially available which detect IgM antibodies produced against Leptospira antigens in the human serum. However, the sensitivity and the specificity vary for different kits in different geographical regions. To date, only two studies have evaluated commercially available rapid diagnostic kits for acute leptospirosis in Malaysia.
One of these studies, evaluated two commonly used rapid leptospirosis serological tests in Malaysia, Leptorapide(Linnodee, Northern Ireland) and VISITECT-LEPTO (Omega Diagnostics, Scotland, UK) reported limited diagnostic value in detecting acute leptospirosis as they showed lower sensitivities and specificities . However, a more recent evaluation study on the IgM Duo Rapid test kit from Korea (immunochromatographic assay), showed a diagnostic sensitivity of 73% and specificity of 90%.
Nonetheless, a point of note regarding the aforementioned studies is that, both of them were performed on previously confirmed MAT/PCR positive samples rather than a prospective clinical evaluation of samples from patients in a hospital setting. As elsewhere, in Malaysia few serovars isolated locally are frequently observed among patients. Hence, it is also important to determine the diagnostic efficacy of any RDT against the locally isolated serovars as well.