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Microprecipitation reaction with cardiolipin antigen
Medical expert of the article
Last reviewed: 04.07.2025
The microprecipitation reaction with cardiolipin antigen for syphilis is normally negative.
The microprecipitation reaction allows to detect antibodies to the cardiolipin antigen of the pale spirochete. The microprecipitation reaction, when used alone, serves not as a diagnostic test, but as a selection test, and therefore, based on its positivity, the diagnosis of syphilis is not established, and the patient undergoes diagnostic tests (RSC, ELISA). The microprecipitation reaction is used to examine persons subject to periodic medical examinations for venereal diseases, patients with somatic diseases, etc.
There are several types of microreactions - VDRL (Venereal Disease Research Laboratory), TRUST (Toluidine Red Unheated Serum Test), RST (Reagin Screen Test), RPR (repid plasma reagin), etc. The RPR test (MPa with cardiolipin antigen) is positive in 78% of cases of primary syphilis and in 97% of cases of secondary syphilis. The VDRL test (MPa with cardiolipin antigen) is positive in 59-87% of cases of primary syphilis, in 100% of cases of secondary syphilis, in 79-91% of cases of late latent syphilis, and in 37-94% of cases of tertiary syphilis. The microprecipitation reaction is usually negative in the first 7-10 days after the appearance of hard chancre.
In case of positive results of VDRL, RPR tests, the titer of reagin antibodies can be determined. A high titer (more than 1:16) usually indicates an active process, a low titer (less than 1:8) - a false positive test result (in 90% of cases), and is also possible in late or late latent syphilis.
The study of antibody titers in dynamics is used to assess the effectiveness of treatment. A decrease in titer indicates a positive response to the treatment. Adequate treatment of primary or secondary syphilis should be accompanied by a 4-fold decrease in antibody titers by the 4th month and an 8-fold decrease by the 8th month. Treatment of early latent syphilis usually leads to a negative or weakly positive reaction by the end of the year. A 4-fold increase in titer indicates a relapse, reinfection, or ineffectiveness of therapy and leads to the need for a repeated course of treatment. In secondary, late, or latent syphilis, low titers can persist in 50% of patients for more than 2 years, despite a decrease in titer. This does not indicate ineffective treatment or reinfection, since these patients remain serologically positive, even if the course of treatment is repeated. It should be taken into account that titer changes in late or latent syphilis are often unpredictable, and assessing the effectiveness of treatment based on them is difficult.
In order to differentiate congenital syphilis from passive carriage of the maternal infection, newborns need to undergo a series of studies to determine the antibody titer: an increase in the titer within 6 months after birth indicates congenital syphilis, while with passive carriage, antibodies disappear by the 3rd month.
When evaluating the results of VDRL and RPR tests in infants with congenital syphilis, it is necessary to remember the prozone phenomenon. The essence of this phenomenon is that for agglutination of antigens and antibodies in these reactions, it is necessary for antigens and antibodies to be present in the blood in the appropriate amount. When the amount of antibodies significantly exceeds the amount of antigens, agglutination does not occur. In some infants with congenital syphilis, the antibody content in the serum is so high that agglutination of antibodies and non-treponemal antigens used to diagnose syphilis does not occur in undiluted serum (the VDRL and RPR tests are nonreactive). Therefore, the prozone phenomenon is possible in children examined for the purpose of diagnosing congenital syphilis. To avoid false negative results in such cases, it is necessary to conduct studies with and without serum dilution.
The VDRL microreaction may be negative in early, late latent and late syphilis in approximately 25% of cases, as well as in 1% of patients with secondary syphilis. In such cases, the ELISA method should be used.
A false-positive microprecipitation reaction is possible in rheumatic diseases (for example, systemic lupus erythematosus, rheumatoid arthritis, scleroderma), infections (mononucleosis, malaria, mycoplasma pneumonia, active tuberculosis, scarlet fever, brucellosis, leptospirosis, measles, mumps, venereal lymphogranuloma, chickenpox, trypanosomiasis, leprosy, chlamydia), pregnancy (rare), in old age (about 10% of people over 70 years of age may have a false-positive microprecipitation reaction), chronic lymphocytic thyroiditis, hemoblastoses, taking certain antihypertensive drugs, hereditary or individual characteristics.
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