Evaluation of rapid diagnostic tests

During the past several years, many new rapid tests for the diagnosis of leptospirosis has been developed. These include a Microcapsule Agglutination Test (MCAT) developed by Central Laboratory, Japan and three tests developed by the Royal Tropical Institute (KIT), Amsterdam, The Netherlands. These three tests are lepto dipstick, lepto lateral flow and lepto dridot test.

MCAT is based on passive agglutination and employs carrier microcapsule particles of a chemically stable synthetic polymer on the surface which ultrasonicated leptospiral antigens are adsorbed. MCAT kit contains two lyophilised reagents, one incorporating antigens of serogroups Australis, Autumnalis and Hebdomadis and the other incorporating that of Canicola, Icterohaemorrhagiae, and Pyrogenes. LEPTO Dipstick uses a broadly reactive leptospiral antigen which detects leptospira specific IgM antibodies in human sera. The Dipsticks have two bands, a lower Leptospira biflexa antigen band and an upper internal control band. The test is performed by incubating dipsticks in a mixture of reconstituted reagent and serum for three hours. The results are read as 1+, 2+, 3+ or 4+ based on the intensity of colour of the antigen band. Lepto lateral flow uses a heat resistant antigen prepared from non-pathogenic leptospira (Pattoc I). The test strip has two bands. If, after adding serum and sample solution to the sample application well, only the control band becomes stained, the test is negative. If both test and control bands become stained the test is considered as positive. The results can be obtained within three minutes. The Lepto Dri - Dot contains colored latex particles activated with a broadly reactive leptospira antigens which are dried on to an agglutination card and is visible as a blue dot. To perform the test 10 ml serum is added on the card near the blue dot, mixed with the dot within the circle. If the test is positive, agglutination becomes visible within a few minutes.

All these tests were evaluated in actual field conditions in South Andaman. Suspected cases of leptospirosis attending primary health centres and hospitals in South Andaman were included in the evaluation studies. The indices of validity and utility of these tests are summarized in table 1.

The test indices changes with the stage of illness. Fig 1. shows the trend in the indices during different stages. Specificity and positive predictive values showed an increasing trend. Sensitivity and negative predictive values peaked during 2nd - 4th weeks and dropped afterwards. Sensitivity during the first week was low. Even during the first week, sensitivity showed an increasing trend with the duration of illness. By about 5th day of illness the sensitivity was cabout 80%. Low sensitivity during the very early phase of the illness is a draw back of all these rapid tests. It is during this period that the clinician needs laboratory support for making a definite diagnosis for initiating early treatment. Although these tests have solved the problem of lack of simple laboratory tests, the efficacy of these tests needs to be improved.