Latex agglutination test for the detection of cysticercus antigen in the urine for the diagnosis of neurocysticercosis

Sir,

Neurocysticercosis (NCC), considered the most common parasitic disease of the central nervous system of humans, is caused by the larva of Taenia solium.[1] Immunological tests are now being increasingly used in adjunct with imaging techniques to aid the diagnosis of NCC. Emphasis has been laid on the demonstration of microbial antigens in various body fluids. Urinary antigen detection is suggested to be an indicator of active infection and is useful for monitoring the efficacy of chemotherapy.[2] Excreted antigens have been demonstrated in urine of patients with a variety of parasitic infections such as chagas disease, visceral leishmaniasis, lymphatic filariasis and malaria.[3,4,5,6] The collection of the cerebrospinal fluid and blood for serum are invasive procedures requiring technical expertise and disposable syringes to prevent the risk of transmission of serious blood borne viral agents. Hence, the use of urine as a clinical specimen in the microbiology laboratory for antigen detection, has gained popularity owing to its numerous advantages. Unlike blood specimens, collection of urine is non-invasive, easy and can be done frequently without causing any inconvenience to the patient.[2] In this context, an attempt was made to evaluate a simple, in-house, latex agglutination test (LAT) for the detection of cysticercus antigen in the urine for the very first time.

A total of 98 urine specimens were collected after obtaining informed consent from all participants or their legal guardians. The study was approved by the institutional research council. The study population was classified into four groups. Group 1 (Clinically and radiologically confirmed with single computed tomography [CT] lesion) included 25 patients with single contrast enhancing ring lesions/single calcified nodular lesions of <0.5 cm diameter on CT scan/magnetic resonance imaging (MRI) presenting with seizures, hydrocephalus or intracranial hypertension or psychiatric disturbances. Group 2 (Clinically and radiologically confirmed with multiple CT lesions) included 23 patients with multiple contrast enhancing ring lesions of <0.5 cm diameter or multiple nodular calcified lesions of <0.5 cm diameter on CT scan/MRI presenting with recurrent generalized tonic clonic seizures and headache. Group 3 (non-cysticercal central nervous systems infection control) included 25 patients presenting with single/multiple margin enhancing lesions/calcified polymorphic lesions of more than 0.5 cm diameter with or without meningoencephalitis or microbiologically proven cases of tubercular or cryptococcal meningitis. Group 4 (healthy controls) included 25 healthy adults (blood donors and students) who had not suffered from cysticercosis or any other disease in the recent past.

Aseptically from each patient, 5 ml of urine was collected and were preserved with sodium azide 0.015 mol/L and stored at −20°C until use. Concentration of urine, preparation of Cysticercus cellulosae complete homogenate antigen and hyperimmune cysticercus antiserum were performed by methods described earlier.[7,8,9] The LAT was designed as described by Devi and Parija and was performed for both unconcentrated and concentrated urine samples.[10] Known positive and negative urine controls were included each time the tests were performed. The results of LAT are summarized in Table 1.

Table 1.

Evaluation of LAT for the detection of urinary antigen in the diagnosis of NCC

When compared with serology for anti cysticercal antibodies (data not shown), the LAT for concentrated urine samples used in the present study, exhibited sensitivity of 58.3% and specificity of 94%. A similar study performed earlier employing co-agglutination to detect cysticercus antigen in the urine of NCC patients had a sensitivity of 62.5% and specificity of 100%.[9] Even though the antigen detection tests are highly specific, the major drawback faced is the low sensitivity which is due to the paucity of detectable antigens released from the cysticerci. In this context, it was observed that the positivity rate of the LAT was higher in patients with multiple cysts (65.2%) than in patients with solitary cyst (52%). Considering the Indian scenario, where single lesion disease is common than multiple lesions, the LAT for antigen detection may not be very useful.[11]

In addition, the LAT was also done for serum samples collected from patients of Group 2 (patients with multiple radiological lesions, data not shown). It was found that the percentage positivity was higher for urinary antigen LAT (65.2%) than that of the serum antigen LAT (47.8%). It was more interesting to find that the LAT for the urinary antigen in patients with single radiological lesion (percentage positivity 52%) performed better than the LAT for the serum antigen in patients with multiple radiological lesions (percentage positivity 47.8%). The most probable explanation for this could be that the antigen in the serum binds to the antibodies and exist as immune complexes while urine contains free antigen, which is more readily detected. These observations suggest that urine could be a better sample than serum for employing tests detecting cysticercal antigens.

Currently, the diagnosis of NCC relies on radiological and serological techniques. A successful antigen detection test, if developed could provide a specific parasitic diagnosis. In an attempt to develop such a test, the present study is the first to use LAT for detection of cysticercal antigen in urine. Although the sensitivity of the test was found to be low, other salient observations were made. NCC antigens were more readily detected when there were multiple cysts. In addition, urine was found to be a superior to serum as a specimen for cysticercal antigen detection. Owing to the merits of urine sample over serum in terms of ease of collection and non-invasiveness, further studies on cysticercal antigen detection could explore the potential use of urine specimens.