Utility of MycoDot test in the diagnosis of cutaneous tuberculosis

Sir,

Mycobacteria are known to have several immunologically active antigenic components like 38 Kd antigen, 30 Kd antigen, 16 Kd antigen, A60 antigen and lipoarabinomannan (LAM).[1] The MycoDot test is a new simple, rapid (20 minutes) and reliable serodiagnostic technique that can detect anti-mycobacterial antibodies in the serum or blood. It offers a low cost, single visit aid in the diagnosis of tuberculosis, with good sensitivity and excellent specificity.

The MycoDot test employs lipoarabinomannan (LAM) antigen which is bound to plastic combs. When the combs are incubated in diluted serum/blood, specific anti-LAM antibodies from the sample, if present, bind to the antigen. The sensitivity of the test is calibrated so that only cases of active mycobacterial diseases such as tuberculosis will generate a colored spot, which is as strong as or stronger than the weakest positive spot on the reference comb that is provided as a guide to interpret results. Healthy infected and/or BCG vaccinated individuals react negatively.

We studied the utility of the MycoDot test in cutaneous tuberculosis by using it to screen the sera of 22 randomly selected patients with cutaneous tuberculosis from 117 cases of cutaneous tuberculosis for the presence of anti-mycobacterial antibodies. All the patients were subjected to routine laboratory investigations and skin biopsy, which revealed epithelioid granulomas. All cases were negative for AFB both in tissue smears and in culture. Of the 22 cases 7 had scrofuloderma, 4 lupus vulgaris, 8 tuberculosis verrucosa cutis, 2 had combination of tuberculosis verrucosa cutis and lupus vulgaris and one patient had erythema nodosum suspected to be due to tuberculosis. All our patients responded well to standard anti-tuberculous treatment.

Out of a total of 22 specimens, the sera from 9 patients tested positive, i.e. 40.8% positivity. The efficacy of the test was 47.8%, and the sensitivity was 43.4%. The negative predictive value was only 7.1%. Hence a positive test is of significance while a negative result does not necessarily rule out tuberculosis.

In Mexico the sensitivity of an ELISA test for IgG antibody to LAM used for the serodiagnosis of tuberculosis was 75% in patients with tuberculosis.[2] ELISA with LAM was also used to detect IgG in the cerebrospinal fluid of patients with tuberculous meningitis.[3] The positivity was 85% and the sensitivity and specificity were 26-81% and 92-100% respectively.

In primary tuberculosis, progression is accompanied by an increase of the antibody titer and healing by a decrease. In secondary tuberculosis, the IgG antibody levels do not return to normal in a fraction of patients. The presence of antibodies merely indicates a past or present mycobacterial infection. However, it has been reported that the presence of antibodies of the IgG type requires the presence of antigen, in the form of active tuberculosis.[4] Hence seropositivity indicates that the disease process is still active in these patients, in the form of antigens being liberated from degenerating bacilli.

Serodiagnosis is not a substitute for clinical examination and histopathology, but only an adjunct in the diagnosis of cutaneous tuberculosis. However, serodiagnosis by the Mycodot test is not adequately sensitive to be useful for screening patients for cutaneous tuberculosis.