T. Pallidum Immobilization Test (tpi)

The TPI possesses the highest specificity among all the existing tests for syphilis. Its principal purpose consists in the disclosure of false positive results of standard (classical) serological reactions. This is particularly important in patients who reveal no clinical manifestations of active syphilis or have lesions of the internal organs or nervous system which may be caused both by a syphilitic infection and by other infectious and non-infectious factors. It would be hard to overestimate the role of the TPI in the recognition of false positive results of standard serological reactions in pregnancy, when the fate (health) of the future child actually depends on the result of the treponema immobilization test.

The essence of the TPI consists in the loss of mobility by T. pallidum in the presence of immobilizins of the serum tested and active complement. The test is conducted under conditions of anaerobiosis. The techniques are rather complicated and the reaction is therefore conducted in special laboratories. Immobilizins appear in the patient's blood serum later than the other antibodies and the TPI therefore becomes positive later than the standard serological reactions and the immunofluorescence test.

The test is considered negative if up to 20 per cent of treponemas are immobilized; weakly negative if 21 to 50 per cent, and positive if 51 to 100 per cent are immobilized. The percentage of treponema immobilization is determined according to a special table.

The result of the TPI is positive in patients with tropical treponematosis (pinta, bejel) and sometimes in sarcoidosis, erythematosis, tuberculosis, cirrhosis of the liver, atherosclerosis and other diseases. With age, the number of false positive results of the TPI grows.

Immunofluorescence Test (ift)

The fluorescence test began to be used in syphilis serodiagnosis from the end of the sixties. It is distinguished from the standard serological reaction by higher sensitivity (that is why in some patients it is positive even in the primary seronegative period of syphilis) with preservation of high specificity. Some clinicians, however, claim that it is inferior to the TPI in specificity and cannot therefore be used instead of this test, though its technique is much simpler. The reaction is performed in more than one modification: IFT-10 (which is more sensitive), IFT-200 and IFT-abs (which are more specific).

The principle of the test lies in that the antigen from the pathogenic treponemas combines with the corresponding antibodies of the blood serum of the syphilitic patient. The complex produced is, in turn, combined with luminescent rabbit serum antibodies against human globulins. In cases with positive results the fluorescein causes yellowish-green fluorescence of T. pallidum seen by means of a luminescent microscope. It the patient does not have syphilis, no fluorescence is demonstrated. The degree of fluorescence is graded by plus signs as in the standard serological reactions: 4+, 3+ and 2+ stand for a positive result, 1+ and absence of fluorescence (—) designate a negative result.