Is the RPR test accurate

Consultation laboratory for Treponema diagnosis of syphilis

Laboratory diagnostics of syphilis

The natural host of the syphilis pathogen Treponema pallidum is humans. The transmission occurs almost exclusively in direct sexual contact through direct contact with infectious efflorescences of the primary and secondary stages. In the asymptomatic phases of early latency, there is hardly any risk of skin or mucous membrane contact due to the low number of persistent pathogens, but there is certainly a risk on the bloodstream. All later stages of infection are distinguished from the potentially infectious early stages as non-infectious.

Tears and sperm are irrelevant as sources of infection. In the case of oral contact (kissing, oral sex), the pathogen can be transmitted if there is a syphilitic lesion at the level of the mouth. Otherwise, saliva is also considered non-infectious.

An indirect transmission of pathogens can be practically ruled out, since T. pallidum is extremely sensitive to all environmental influences, in particular to temperature fluctuations, drying out or the effects of detergents.

Direct evidence

Fig. 6 Treponema pallidum in dark field microscopy, stained using the immunofluorescence technique
Source: © CDC / C.W. Hubbard

The direct detection of the pathogen is generally only possible in the early stages of syphilis from the irritant secretion of the primary lesion or the efflorescences of the secondary stage using dark field microscopy (Fig. 6). The examination must take place immediately after the material has been removed. The test evaluation requires a lot of practical experience. Samples from the gastrointestinal tract or the oral cavity are not suitable.

Alternatively, a cotton swab smear (without preservatives) from all parts of the body can be examined in the special laboratory using treponemic PCR. However, the diagnostic value of this time-consuming and expensive examination technique has not been conclusively clarified. In principle, tissue samples, blood, cerebrospinal fluid or aqueous humor can also be examined with the treponema PCR, but this should be reserved for special questions. It is important to note that no direct or indirect negative pathogen detection excludes syphilis.

Method of choice

The method of choice for laboratory diagnosis of syphilis is the detection of antibodies from whole blood or serum. If neurosyphilis is suspected and there are positive antibodies in the serum, a serum and liquor sample taken in parallel must be examined. Since the treponemic antibodies are very stable in blood and liquor, sending samples is unproblematic and corresponds to the procedure for other serological tests. Proper sample labeling, information on the current clinical issue and, if applicable, on previous findings or the treatment history are important. This is crucial for the selection of suitable tests in the laboratory and, above all, for the evaluation of findings.

test principleSensitivity (%) Specificity
%


Early stage Late stage
RPR-VDRL RPR = Rapid Plasma Reagin
VDRL = Venereal Disease Research Laboratory
Detection of cardiolipin
Antibodies,
not pathogen-specific, quantitative
50-85 untreated: 60-96
treated: 6
97
TPPA Treponema Pallidum Particulate
Agglutination test;
pathogen-specific, quantitative
60-100 untreated: 100
treated: 96
>99
IgG + IgM search test T. pallidum specific IgG + IgM;
pathogen-specific, semi-quantitative
98 untreated: 100
treated:> 98
>99
IgG immunoblot T. pallidum specific IgG;
pathogen-specific, qualitative
95 99 >99
IgM immunoblot T. pallidum specific IgM;
pathogen-specific, qualitative
90 untreated:
64 -> 20
congenital: 80
>99

Fig. 7 Serological tests

The concept for step-by-step diagnostics established in Germany, consisting of a search test, confirmation test and tests to assess the disease activity and, if necessary, the need for treatment, has proven itself in practice for many years. The established screening tests for the detection of T. pallidum-specific total antibodies (IgG / IgM) are the Treponema pallidum particle agglutination (TPPA) and the Treponema pallidum hemagglutination (TPHA) test. In principle, both tests provide comparable results. When using different test products, however, titre differences of several dilution levels can result. The round robin tests for external quality control show that the titers can vary considerably from laboratory to laboratory even if the same test is used. Therefore, titer controls for these tests should always be carried out in the same laboratory if possible. Alternative syphilis screening tests based on recombinant treponemal antigens are increasingly being used in routine diagnostics. These polyvalent enzyme or chemiluminescence assays are comparable to the TPPA and TPHA tests in terms of sensitivity and specificity. The TPPA and TPHA tests seem to react somewhat more sensitively only for the detection of low-positive residual antibody findings (sero scars) (Fig. 7).

Rapid test

Reliable statements on sensitivity are not yet possible for the syphilis rapid tests that have recently been increasingly propagated. Syphilis rapid tests are not recommended by the specialist committees, e.g. the German STI Society, the RKI or the Paul Ehrlich Institute, but also by the WHO.

Ser conversion


Fig. 8 Serological course of syphilis

The syphilis screening tests become positive approx. 2-3 weeks after the infection, and then usually remain positive for many years, mostly for life. The treponema-specific antibodies can only drop below the detection limit again with very early therapy (Fig. 8).

The fluorescence treponema antibody absorption (FTA-ABS) test is accepted worldwide as a syphilis confirmatory test. Alternatively, routine laboratories use immunoblots. Here, too, the sensitivity is not always sufficient, with the result that positive addiction test results can also be misinterpreted as unspecific. According to the more recent diagnostic recommendations, immunoassays can also be used for TPPA / TPHA screening and, vice versa, the TPPA / TPHA test can also be used as confirmatory tests for immunoassay screening. This has the advantage that the TPPA / TPHA titer can also be subsequently determined during screening with EIA or chemiluminescence assays, which is helpful for the interpretation of the findings. If the search and confirmation tests are positive, they are considered evidence of a specific immune response against T. pallidum regardless of the possible stage of infection.

Assessment of the activity

The pathogen-specific IgM antibodies (19S IgM FTA ABS test, IgM immunoblot, IgM EIA) and the non-specific lipoid antibodies (VDRL , RPR, Cardiolipin-KBR) can be determined quantitatively. Specific IgM and non-treponemal lipoid antibodies are not to be seen as alternatives but as complementary test methods. The RPR (Rapid Plasma Reagin) test is a modification of the VDRL test. The titers determined with both methods are comparable. The Cardiolipin KBR usually results in higher titer values. The evaluation suggestions for lipoid antibody titers relate worldwide to the VDRL or RPR test, but not to the Cardiolipin KBR.

In the first syphilis infection, high IgM antibody titers are found at an early stage with initially negative lipoid antibody findings. In late latent infections and late stages of syphilis, the specific IgM antibody result can be negative, while lipoid antibodies can still be detected. The syphilis reinfections, which are relatively common today, usually have high lipoid antibody titers, while the specific IgM antibody result can vary from negative to highly positive. The IgM antibody finding can also vary depending on the method. The 19S-IgM-FTA-ABS test yields more positive results than the IgM-EIA and IgM immunoblot. It is important to remember that a negative IgM antibody finding does not fundamentally rule out active syphilis that requires treatment! The laboratory results must always be interpreted in the context of the anamnesis and current clinical situation of the individual patient.


Prof. Dr. med. Hans-Jochen Hagedorn
National consulting laboratory for Treponema (diagnosis and therapy) Siemensstr. 40 32105 Bad Salzuflen, Germany
E-mail: [email protected]


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