VDRL Test and its Interpretation

Abstract

Venereal disease research laboratory (VDRL) test is a nontreponemal test, used for screening of syphilis due to its simplicity, sensitivity and low cost. Prozone phenomenon and biological false positive (BFP) reaction are two shortcomings of this test. Quantitative estimation of VDRL is essential in treatment evaluation. CSF VDRL test is very specific for neurosyphilis though its sensitivity is low. Interpretation of VDRL in HIV infection is incompletely understood.

Introduction

Since very long time Venereal Disease Research Laboratory (VDRL) test is performed solely by physicians to screen patients for syphilis, yet VDRL is still the most commonly used test all over the world for screening and it still remains unchallenged. Most interesting fact is that it is a test most commonly performed by patients themselves, even in the absence of a medical advice and the result obtained thereof is remembered and reminded to consulting physicians every time they visit them for rest of their life. This holds true when a patient suffers from a disease pertaining to genital system. However, most horrifying fact is that a positive test penetrates so deep into patients’ psyche that patient is self-stigmatized and hurt affecting him mentally and physically, affecting his daily activities and performance. Clinically in immunocompetent persons, a negative result is strong evidence against presence of the disease. In human immunodeficiency virus (HIV)-infected persons, the serological response is unusual as in many other diseases, so while interpreting a serological test result, great care should be given. The aim of this article is to convey the right massage to physicians and patients about the facts and misfacts surrounding this test and importance of proper interpretation of its results.

The Test

There are three basic methods used in screening for syphilis. These include direct observation of the spirochete by dark field microscopy, and nontreponemal and treponemal serologic antibody studies. More sensitive nontreponemal tests such as the rapid plasma reagin (RPR) and the VDRL are used for initial screening, whereas specific treponemal tests such as the fluorescent treponemal antibody absorption (FTA-ABS) are used to confirm the diagnosis. The importance of these screening tools is shown by past clinical studies that have demonstrated 78% accuracy for the clinical diagnosis of primary syphilis by experienced clinicians.[1] This is a nonspecific test but it is useful in following treatment, since the antibody titer declines on successful therapy. Nontreponemal tests are rapid, simple, and inexpensive. They are the only tests recommended to monitor the course of disease during and after treatment. Nontreponemal tests can also serve to detect reinfection. The main limitations of nontreponemal tests are their reduced sensitivity in primary syphilis and late latent syphilis, false-positive results due to cross reactivity, and the potential for false-negative results due to prozone phenomenon. Unfortunately, no current laboratory test can distinguish one trepanomatosis from another, and this must be considered in serology in areas of the world where yaws, pinta on endemic trepanomatosis exist.

The basis of the VDRL test is that body produces antibody when infected, and in this test the antibody is detected by subjecting the serum to an antigen, which is composed of colorless alcoholic solution of beef cardiolipin, cholesterol, and lecithin. It is a qualitative test for screening of syphilis, and currently all nontreponemal tests are flocculation tests and both VDRL and RPR tests are modifications of original Wasserman reaction. Unfortunately in this test, the antibody detects antigens, which are nonspecific, thereby yielding much false-positive reaction.

In this test, heated serum or unheated cerebrospinal fluid is mixed with reagin (a purified mixture of lipids such as cardiolipin, lecithin, and cholesterol) on a glass slide, and flocculation, or clumping, of the mixture is read microscopically as “reactive” (if clumping occurs) or “nonreactive” (if there is no clumping). Like the RPR test, the VDRL test can be quantitated by examining serial dilutions of serum and can be used to follow the course of illness, including the response to therapy.

Interpretation

In a healthy person, the test is negative. This means that no antibodies to the organism that causes Treponema pallidum/bacteria. As antigen used in nontreponemal test is component of all mammalian cell membranes, the damage to host tissue by infection, immunization, pregnancy, age-related changes, or autoimmune diseases can result into false-positive nontreponemal test results.[2,3] Even if the test is positive, one cannot certainly give an opinion in regards to the infective status of the patient with syphilis. Therefore, without taking a good history and conducting some other tests to exclude other diseases and scenarios where it can yield false-positive test phenomena referred to as biological false-positive (BFP) VDRL test and incidence is generally 1–2%. Serologic tests provide only indirect evidence of syphilis and may be reactive in the absence of clinical, historical, or epidemiologic evidence of syphilis. The VDRL usually becomes reactive within the first few weeks after infection, peaks during the first year, and then slowly declines, so that low titers (levels) are seen in late syphilis. It can revert to negative in the absence of treatment in about 25% of cases. The sensitivity of nontreponemal and treponemal tests for syphilis increases with duration of infection, and ranges from approximately 75% in the primary stage to virtually 100% in the secondary stage.[4]

Serologic tests provide only indirect evidence of syphilis and may be reactive in the absence of clinical, historical, or epidemiologic evidence of syphilis. The reactivity in such cases is usually in low dilutions (<1:8), however, in exceptional cases false reactivity in very high titers (up to 1:256) has been reported;[5] therefore, quantitative titer cannot be used to differentiate between a false-positive reaction and syphilis. This is especially true for persons who are IV drug users, where more than 10% have false positivity with titers >8. False-positive reactions can also occur with treponemal tests, but this is less common than with nontreponemal tests Therefore, careful clinical interpretation of test results and other evidence is necessary for proper diagnosis. Besides being BFP reaction, another factor needed to be look into is reliability of technique and the quality of the laboratory where it is done, because incidence of positivity increases due to faulty procedure or wrong interpretation. BFP reaction is for inherent reasons and not due to any technical error. BFP reactions comprise a high proportion of all VDRL reactors. Therefore, the use of the VDRL as a screening procedure is challenged. Therefore, before giving any conclusive opinion in regards to VDRL reactivity, we need to take a proper history regarding any chronic illness such as, systemic lupus erythematosus, rheumatoid arthritis or any autoimmune or collagen disease, antiphospholipid syndrome, drug addiction, liver disease, or malignancy where there is every possibility of chronic biological false positivity. Besides many febrile illnesses such as malaria, tuberculosis, filariasis, and conditions such as pregnancy, aging, and immunization yield a transient false positivity. Biological false positivity warrants a repeat of syphilis serology. It is recommended that serology should be repeated at 10 weeks, as by that time most cases will return to VDRL nonreactivity.[6] Therefore, we need to co-relate test report with clinical conditions and history to give a useful valid report. Another important thing that is needed to be looked into is, Pro-zone phenomenon: Prozone reactions are false-negative reactions that occur due to interference by high concentrations of target antibodies in a specimen. The disproportionate antibody-to-antigen ratio results in a “rough” nonreactive or a very weakly reactive reaction. Such specimens will give a clearly positive reaction when diluted and retested, a process that brings the antibody-to-antigen ratio within the optimal range. The zone of equivalence defines this optimal ratio. In the eve of antibody or antigen, excess (prozone and postzone, respectively) false-negative test result will arrive. Therefore, it is advisable and recommended to test antibody for prozone phenomenon which is preferred by diluting patient's result to bring antibody concentration into the “zone of equation.” However, the incidence of prozone phenomenon is very low in non-HIV patients with syphilis, ranging from 0% to 0.4%.[7] It may occur in the very late stage of the disease.

Nontreponemal screening tests have a sensitivity of 70–90% in primary syphilis. It needs to be confirmed by a treponemal test. All serological tests are positive at secondary stage and sensitivity for all tests including VDRL test which is approximately 100%; however, in 1–2% of patients’ false-negative nontreponemal tests can occur due to prozone phenomenon. A presumptive diagnosis is based on the presence of typical rash and reactive non-treponemal tests in a titer ≥1:8 in a patient with no previous history of syphilis. If history of syphilis is present, then the criteria should be a fourfold rise in titer. When a titer of nontreponemal test <1:8, the test should be repeated and a treponemal test should also be performed.

All serological tests are reactive in the early latency; however, the reactivity of the nontreponemal tests decreases with the increasing duration of latency, and in approximately 30% patients with late latent or late syphilis VDRL/RPR test are negative. In all patients with late latent syphilis, lumbar puncture is recommended to exclude neurosyphilis.[8] In low prevalence populations, false-positive results are common with both treponemal and nontreponemal tests.

As in up to 30% patients nontreponemal tests may not be reactive, treponemal tests should be performed. However, 2–4% of patients with late syphilis will show negative results with treponemal tests also. As the number of organisms are too low in late stages, polymerase chain reaction (PCR) technique may be a very useful test in establishing a diagnosis.

Pregnancy

Optimally pregnant women should be screened for syphilis at their first prenatal visit, during third trimester, and at the time of delivery with a nontreponemal test (VDRL or RPR). A confirmatory FTA-ABS test should be performed if VDRL or RPR is positive near delivery. For mothers with:

1. Positive VDRL or RPR at the time of delivery and/or

2. A history of syphilis which is untreated, and/or

3. A treatment history which cannot be documented.

The infant should receive an evaluation for clinical signs and symptoms of syphilis. This evaluation must include a nontreponemal serological test performed on infant serum, not cord blood. Titers may increase slightly in serofast women who were previously treated for syphilis become pregnant. It is generally less than fourfold increase. Serological response to treatment is similar to that of non-pregnant women.

Congenital Syphilis

The diagnosis of congenital syphilis is complicated by the trans-placental transfer of maternal nontreponemal and treponemal IgG antibodies to the fetus. This transfer of antibodies makes the interpretation of reactive serologic tests for syphilis in infants difficult. Treatment decisions frequently must be made on the basis of (1) identification of syphilis in the mother; (2) adequacy of maternal treatment; (3) presence of clinical, laboratory, or radiographic evidence of syphilis in the infant; and (4) comparison of maternal (at delivery) and infant nontreponemal serologic titers using the same test and preferably the same laboratory. Venous blood from both the mother and the child should be tested. Asymptomatic congenital syphilis requires a comprehensive approach. All infants born to mothers who have reactive nontreponemal and treponemal test results should be evaluated with a quantitative nontreponemal serologic test (RPR or VDRL) performed on infant serum because umbilical cord blood can become contaminated with maternal blood and could yield a false-positive result and should be examined thoroughly for evidence of congenital syphilis (e.g., nonimmune hydrops, jaundice, hepatosplenomegaly, rhinitis, skin rash, and/or pseudo paralysis of an extremity). Conducting a treponemal test (i.e., TP-PA or FTA-ABS) on a newborn's serum is not necessary.

Neurosyphilis

According to Centers for Disease Control and Prevention (CDC), a person with any of the following criteria should have CSF examination.[9]

1. late latent syphilis/latent syphilis

2. who has treatment failure as defined by serological titer response

3. who is HIV infected, or who has active tertiary syphilis.

Standard parameters of neurosyphilis on CSF examination include mononuclear cell count greater than 5–10 cells/mm³, protein concentration greater than 40 mg/dL, and a reactive CSF-VDRL.[3] Although regarded as the gold standard for the diagnosis of neurosyphilis, the VDRL-CSF, the standard serological test for CSF, is highly specific but is not 100% sensitive, may be negative in up to 50% of samples from patients with neurosyphilis.[10,11] Therefore, a negative VDRL-CSF result does not rule out neurosyphilis. A reactive CSF-VDRL test, free of blood or other contaminants, usually indicates past or present syphilis infection of the central nervous system (CNS). A BFP test result is rare in spinal fluid. A nonreactive VDRL test may indicate that the patient does not have neurosyphilis. However, a negative result can occur in some patients with neurosyphilis. CSF treponemal test have high sensitivity and are helpful only when test is negative. A nonreactive test probably excludes neurosyphilis, but a positive test is not always diagnostic for neurosyphilis. However, in a patient co-infected with HIV and syphilis, it is difficult to diagnose neurosyphilis on the basis of CSF changes. In this case, testing of deoxyribonucleic acid (DNA) by PCR is an evolving technique that may be helpful. A reactive CSF-VDRL test or abnormal CSF indices that cannot be attributed to other ongoing illness requires re-treatment for possible neurosyphilis.

To limit unnecessary VDRL-CSF tests, a patient's serum should be reactive in a treponemal test before being accepted for VDRL-CSF testing. The fluorescent treponemal antibody-absorption double staining [FTA-ABS (DS)] test is used for confirmation of the present or past infections. FTA-ABS (DS) test is performed only if specifically requested, and serum shows some degree of reactivity to a nontreponemal test. For patients with neurosyphilis, repeat serologic testing and CSF examinations at 6-month intervals are recommended until the findings have stabilized. Abnormal white blood cell count and protein level in the CSF should decrease by 6 months if no coexisting CNS infections are present, but CSF-VDRL test results may remain reactive for at least 2 years. If the CSF white blood cell count is not normal or the CSF-VDRL remains reactive at 2 years, and if no other cause is identified, then the patient should be re-evaluated and re-treated for neurosyphilis. In conclusion, it seems that the Treponema pallidum particle agglutination technique (TP.PA) can be used in CSF to diagnose neurosyphilis, although as for other serological tests, interpretation of results should be done in conjunction with other neurosyphilis parameters.

HIV and VDRL Test

The interaction between syphilis and HIV infection is complex and remains incompletely understood, despite there being more than two decades of clinical experience with co-infected patients.

Serologic tests for syphilis are the cornerstone of diagnosing untreated syphilis infection, independent of HIV status. Nontreponemal assays, such as the RPR card or VDRL test, use cardiolipin-, lecithin-, and cholesterol-containing antigen to measure antilipoidal antibodies and are often used initially to diagnose syphilis. The sensitivity of nontreponemal and treponemal tests for syphilis increases with duration of infection, and ranges from approximately 75% in the primary stage to virtually 100% in the secondary stage. Because the sensitivity of nontreponemal tests is lower that that of treponemal tests in the primary stage, a negative nontreponemal test in an HIV-infected individual with a genital lesion cannot exclude primary syphilis. It may be useful to consider both a nontreponemal and a (nonreflexed) treponemal test as a diagnostic strategy in newly infected persons with suspicious lesions. Unusual serologic responses have been reported in HIV-infected persons with syphilis. Most reports involved higher than expected serologic titers, but false-negative serologic results and delayed appearance of sero-reactivity have also been reported, albeit rarely. Nevertheless, serologic tests appear to be accurate and reliable for the diagnosis of syphilis and the evaluation of treatment response in most HIV-infected patients. The clinician should seek confirmatory evidence for the diagnosis from any available source, including the patient's history, clinical findings, direct examination of lesion material for spirochetes, and serologic tests for syphilis. Reports of nontreponemal antibody test results should be quantitative and describe the lowest dilution (i.e., the titer) at which the test result is reactive. In general, nontreponemal test titers may be higher among HIV-positive patients than among HIV-negative persons.[12]

The interaction of syphilis and HIV infection is complex and remains the subject of ongoing research. Although case reports have suggested that coexisting HIV infection may alter the natural history of syphilis, only a few such effects have been demonstrated in large observational studies.[13,14] The diagnosis of syphilis may be more complicated in the HIV-infected patients. Initial serologic responses to early syphilis were shown to be generally equivalent in HIV-negative and HIV-positive patients.[15] Reports of false-positive and false-negative results on serologic tests for syphilis in HIV-infected persons raise questions regarding the specificity and sensitivity of serologic diagnoses in such patients.[16,17] Unusual serological responses have been reported in syphilis, namely, higher than expected serologic VDRL titers,[12,18–20] but false-negative serologic result or delayed appearances of seroreactivity have also been reported. Nevertheless, both treponemal and nontreponemal serologic tests for syphilis are accurate in the majority of patients with syphilis and HIV co-infection.[20] The incidence of the prozone phenomenon could be expected to be higher in the HIV-infected population and always recommended to test for prozone phenomena.[21] The diagnosis of neurosyphilis in the individual infected with HIV can be difficult. The sensitivity of the CSF-VDRL can vary widely, and the treponemal tests (microhemagglutination (MHA) assay for T. pallidum (MHA-TP) and the FTA) lack specificity for neurosyphilis.

Irrespective of sign and symptoms, all HIV-positive patients should have baseline VDRL screening and follow-up at 3 months to rule out the possibility of false-negative results, as seroconversion generally takes about 4–6 weeks after infection. In the course of reactive VDRL, the diagnosis of syphilis should be confirmed by the specific treponemal test, namely FTA-ABS or TPHA should be done along with VDRL. Reactive minimal reaction in the FTA-ABS (DS) test provides evidence for or against syphilis. Beaded reaction in the FTA-ABS (DS) is most often associated with chronic inflammatory diseases. Repeat testing is indicated in both instances. If test results are still equivocal, a MHA test may be requested from CDC.

HIV is considered to be one of the important causes of false-positive reaction to syphilis serology; but this conception needs to be reinterpreted as most of the studies on syphilis serology in HIV positives have used populations that include those involved in IV drug use, a behavior that is an independent risk factor for false-positive serology. Further prospective study on the HIV-infected patients with biological false-reactive VDRL results to assess the seroconversion pattern and possible silent abnormality is recommended.

VDRL and Treatment Follow-up

There is no ideal test of cure of syphilis available that can be carried out within days or weeks after treatment to determine the status of a patient. Patients should have serological tests for syphilis done on the day treatment is initiated. To access it, the patient is asked to repeat quantitative nontreponemal serological testing like VDRL/RPR and clinical evaluation at 3, 6, and 12 months. When serological test is negative, patient is assured to be cured. Generally, seropositivity is achieved in majority of the patients with primary syphilis in about 12 months after the treatment and in those with secondary syphilis in about 24 months. Seroconversion is more rapid after therapy if duration of infection is short and initial titer is low. As seroconversion is a slow process requiring months to years, the rate of decline is a better indicator of therapeutic response. A 4-fold decrease in titer is considered as good response, and this should occur within 3-6 months after therapy in patients with primary and secondary syphilis and within 12 months in patients with early latent syphilis. The VDRL titer may not decrease in patients with late syphilis and remains reactive at a low level (<1:8) for many years after adequate treatment. There is no satisfactory monitoring test available for nontreponemal test-negative late disease.[10]

Nontreponemal tests are then monitored at a 6-month interval in patients with latent disease without neurosyphilis to document a fourfold decrease/seronegativity within 24 months after treatment. Low titers will persist in approximately 50% of patients with late syphilis after adequate therapy after 2 years.[22] This low titer persistent seropositivity does not signify treatment failure or reinfection, and these patients are likely to remain serofast even if they are retreated. The FTA-abs (DS) is not recommended for treatment follow-up. Nonreactive serologic tests and normal clinical evaluation cannot exclude incubating syphilis.

In patients with neurosyphilis VDRL-CSF is repeated at 4- to 6-month interval, if CSF pleocytosis or CSF-VDRL titers are noted initially. Some experts recommend that HIV-infected patients be followed more closely at 3-month interval.

The literature VDRL reactor values for treated late latent syphilis and treated late manifest syphilis are 1% only.[23] Although higher values may have been reported, they were unaccompanied by a specification as to the interval lapsed between the times of treatment and testing.[24] Moreover, not even untreated latent syphilis or late syphilis exceeds the VDRL reactivity of 70% (30% of the diseased spontaneously become negative).

The criteria for treatment failure currently include the following findings:

Persistence, recurrence, or development of clinical signs or symptoms of syphilis in the absence of reinfection.

Sustained (greater than 2 weeks) fourfold (two dilutions) increase in the nontreponemal test titer (assuming the same test type was used [e.g., VDRL, RPR].

Failure of the initial nontreponemal test titer to decrease fourfold (two dilutions) by 6–12 months for primary, secondary, or early latent syphilis and by 12–24 months for late latent syphilis or syphilis of unknown duration.

Conclusions

Syphilis has been referred to as the great imitator due to its wide variety of clinical presentations. Therefore, prompt diagnosis and treatment are essential not only to lower transmission rates, but also to avoid the complications seen in the later stages of the disease. However, when test result that is to be conveyed to the patient in a community where a limited resource is available, we need to inform the patient that a definitive diagnosis is only possible by performing, dark field examinations and direct fluorescent antibody (DFA) test. VDRL is just one of the tests to make an presumptive diagnosis. The use of only one type of serologic test is insufficient for diagnosis, because false-positive nontreponemal test results are sometimes associated with various medical conditions and situations unrelated to syphilis as mentioned above. T. pallidum may be visualized using dark ground microscopy, but the presence of spirochetes may be difficult to demonstrate, particularly if the patient has received recent antibiotic therapy. It is not easily cultured, and cannot grow on artificial media. Therefore, the diagnosis of infection is commonly performed using the VDRL and TPHA tests. This is a nonspecific test but it is useful in following treatment, since the antibody titer declines on successful therapy. It also impresses on nonvenereologists the need to cautiously order and interpret serological tests for the demonstration of syphilis. This subsumes quantifying the VDRL among the infected, and the use of modern tests to determine the intensity of syphilitic process. In general, the sensitivity of treponemal tests continues to approximate 100% in late syphilis, in contrast to nontreponemal tests, which are more practical and cost-effective for initial screening but have diminished sensitivity in late syphilis. Despite the higher sensitivity of treponemal tests, they have not been recommended for initial screening in the many countries, primarily because of cost.

It is always advisable to interpret properly before giving any diagnosis to avoid any legal complications in future, so also for the psychological well-being of the patient. In accord with all diagnostic methods, a final diagnosis should not be made on the basis of result of a single test, but should be made on co-relation of test results with other clinical findings.