A cause of false-positive result in the rapid plasma reagin (rpr) test for syphilis is:

Serum nontreponemal antibody tests

The rapid plasma reagin (RPR) and the Venereal Disease Research Laboratory (VDRL) are the standard nontreponemal tests. The RPR measures macroscopic agglutination of cardiolipin-coated charcoal particles by unheated serum; it is sensitive, but not specific, and more useful for screening and to assess the adequacy of therapy. The VDRL measures microscopic agglutination of cardiolipin and lecithin-coated cholesterol particles by heat-inactivated serum into a slide. The RPR and VDRL perform similarly with sensitivity of 70% for the primary, 99% for secondary, and >90% for tertiary forms. False negatives can be common during the first trimester of pregnancy (Tramont, 1995) and do occur in secondary syphilis due to the prozone phenomenon or in those with HIV infection (Schofer et al., 1996). In general, any strong acute bacterial or viral infection, or vaccination, can transiently (<6 months) falsely elevate nontreponemal serology (Tramont, 1995). Many causes of false-positive nontreponemal serologies have been reported, including systemic lupus erythematosus, rheumatoid arthritis, pregnancy (Salo et al., 1969), injection drug abuse, aging, hypergammaglobulinemia, multiple blood transfusions, chronic liver disease, malaria, leprosy, infectious mononucleosis, HIV infection (Malone et al., 1995), relapsing fever, yaws, pinta, leptospirosis, and rat bite fever; false-positive nontreponemal serologies rarely have titers >1:8 (Lukehart and Holmes, 1991). While the frequency of false positives is <2% when screening populations at risk for syphilis (Lukehart and Holmes, 1991), it can increase up to 11% when screening also includes HIV-infected patients (Terry et al., 1988). HIV-infected patients are also more likely to show increasing titers after adequate therapy (Brown et al., 1985; Hutchinson et al., 1991). In most cases, nontreponemal serology becomes negative with treatment, although a persistently low titer reactive serology is common after treatment of neurosyphilis of more than 1 year’s duration.

The nontreponemal titer is highest during early infection and lowest in latent and tabetic syphilis (Stokes et al., 1944; Hahn et al., 1956). When the diagnosis of syphilis is in doubt, it is assumed that high titers are of greater diagnostic significance than low titers (Stokes et al., 1944). Titers tend to be higher in HIV-infected patients (Gourevitch et al., 1993; Malone et al., 1995). However, reagin titers in the blood do not necessarily correlate with disease severity. High titer of blood or CSF nontreponemal serology, with moderate to markedly increased CSF leukocytosis, is characteristic of parenchymatous neurosyphilis. Most positive nontreponemal serologies become nonreactive after successful treatment in 6 months to 2 years in early syphilis (Fiumara, 1980; Brown et al., 1985; Marra et al., 1996b) and by the fifth year in late syphilis (Fiumara, 1979). Persistently positive nontreponemal serologies at high titer after treatment are more frequent in patients infected with HIV (Rampalo et al., 1992; Rolfs et al., 1997; Yinnon et al., 1996).

Problems in the serological diagnosis of syphilis

Occasionally, both the non-specific and specific tests produce false-positive results. The RPR and VDRL assays may give a transient positive result following any strong immunological stimulus such as acute bacterial or viral infection or after immunization. More persistent false-positive results occur in individuals with autoimmune or connective tissue disease, in drug abusers and in individuals with hypergammaglobulinaemia. False-positive results usually become apparent when negative results are found in specific serological tests, but in some cases FTA-Abs results may also be positive or borderline.

Rarely, the FTA-Abs test may be positive and the nonspecific VDRL test negative. Lyme disease (see below) induces antibodies that react in the FTA-Abs but not in the VDRL assay. Other spirochaetal diseases such as relapsing fever, yaws, pinta and leptospirosis may give positive results in both specific and non-specific tests. Of particular difficulty is the differential diagnosis of syphilis and yaws in immigrants from areas in which yaws is endemic.

Some of the newer enzyme immunoassays are less sensitive in cases of primary syphilis.

Direct detection of spirochaetal deoxyribonucleic acid (DNA) in clinical material by molecular methods, such as the polymerase chain reaction (PCR), may have a future role in confirming a diagnosis of syphilis in difficult or atypical cases.

Diagnosis

Serologic tests that have been used include the nontreponemal antigen tests (Venereal Disease Research Laboratory (VDRL) and rapid plasma reagin), microhemagglutination, and fluorescent treponemal antibody absorption tests (DiGiacomo et al., 1983). Although the nontreponemal antigen tests were not completely satisfactory, the VDRL test was more ­sensitive and the plasma reagin test was more specific in detecting T. paraluiscuniculi infection (DiGiacomo et al., 1983). The sensitivity and specificity of the microhemagglutination test compared favorably with the ­fluorescent treponemal antibody absorption test and was recommended as the optimal assay to make a diagnosis (DiGiacomo et al., 1983). Detection of T. paraluiscuniculi in lesions can be achieved by dark-field microscopic examination of scrapings from lesions and by histological evaluation of silver-stained testicular sections (Cunliffe-Beamer and Fox, 1981a; Faine, 1965). PCR has been used for ­molecular characterization of treponemes including T. paraluiscuniculi (Cejkova et al., 2013).

Monitoring Therapy

Determining the adequacy of therapy depends on careful follow-up of the patient. Conversion of the serum VDRL or rapid plasma reagin (RPR) to nonreactive should occur within 1 year after treatment of primary syphilis, within 2 years after treatment of secondary syphilis, and within 5 years after treatment of latent syphilis. This delay to reversion from a seropositive status reflects the duration and severity of the illness. Persistent seropositivity suggests persistent infection, reinfection, or a biological false-positive test.

The fixed neurological deficits of neurosyphilis may fail to improve with treatment, and some abnormalities, such as tabes dorsalis and optic atrophy, may worsen despite adequate therapy. Resolution of CSF abnormalities is the best determinant for the adequacy of treatment. Examination of the CSF within several days of the institution of penicillin treatment may be misleading because the CSF cell count may increase initially, particularly if accompanied by a Jarisch–Herxheimer reaction. However, the CSF should be examined at the termination of treatment to document a decrease in cell count, and it should then be examined at 6-month intervals for 2 or 3 years. The cell count should return to normal within 1 year of treatment (usually 6 months) and the protein concentration within 2 years. The disappearance of the CSF VDRL typically parallels its resolution in the serum and may be a less useful parameter of treatment adequacy than CSF cell count or protein concentration. Importantly, the CSF VDRL titers should not increase over time following effective therapy.

The potential for relapse of neurosyphilis following a course of recommended therapy suggests the potential need for secondary prophylaxis in treating neurosyphilis in the HIV-infected individual, as is employed in the management of some other CNS infections such as toxoplasma encephalitis and cryptococcal meningitis. In a study of 100 HIV-infected military personnel with syphilis, 4 of 7 persons with reactive CSF VDRL relapsed following high-dose intravenous penicillin. These relapses were often observed more than 12 months after initial therapy. However, these findings among HIV-seropositive patients have not been universally observed. The CDC has recommended that the initial therapy of intravenous aqueous penicillin be followed in HIV-infected individuals by weekly intramuscular injections of 2.4 million units of benzathine penicillin for 3 weeks. An alternative therapeutic course is the administration of a 30-day course of 200 mg doxycycline twice daily following the completion of intravenous therapy. Although secondary prophylaxis is extensively employed, further studies are warranted before secondary prophylaxis or some permutation of it can be broadly recommended. HIV-seropositive patients should be carefully monitored for relapse of neurosyphilis for two or more years following initial treatment.

Diagnosis

Diagnosis of VM is by exclusion and requires ruling out other causes by measuring serum vitamin B12 levels, copper levels, rapid plasma reagin (RPR), and human T-lymphotrophic virus-1 (HTLV-1) antibodies before making the diagnosis. Lumbar puncture should be performed to rule out infection with herpes simplex virus, varicella zoster virus, CMV, and neurosyphilis. In VM the CSF may be normal, or it may show mild pleocytosis with mild elevation in protein. Similar CSF findings may be seen in asymptomatic HIV persons. Therefore, CSF is not helpful in making the diagnosis but remains important in excluding the above noted infections. The spinal MRI may be normal, or it may show spinal atrophy or patchy abnormalities on T2-weighted images. Somatosensory evoked potentials (SEP) help to confirm myelopathy, whereas nerve conduction studies can detect coexistent neuropathy if present.

Prognosis and Follow-Up

The lesions become noninfectious within 24 hours of treatment. Whereas treatment in early stages should result in cure in almost 100% of patients, it will not reverse any destructive change in late stages. RPR (or non-treponemal antibody test) titers should decline within 6–12 months, becoming negative in about 2 years. However, in a small proportion of cases, especially if treated in late stages, the RPR (or non-treponemal antibody test) may remain positive, albeit in low titer (below 1 : 8). The specific tests (i.e. TPHA, TPPA, FTA-ABS, rapid treponemal tests) will remain positive throughout life.

Special Tests

The following tests are indicated in the routine workup of a patient with dementia: laboratory studies, including complete blood cell count and measurements of thyroid-stimulating hormone; measurement of vitamin B12 levels and rapid plasma reagin; and neuroimaging (computed tomography or magnetic resonance imaging). Other tests may be ordered as the clinical situation dictates. It is not necessary to perform a lumbar puncture in the routine assessment of a patient with Alzheimer's disease,22,23 but it is indicated when the following diagnoses are suspected: central nervous system infection, prion disease, hydrocephalus in the presence of normal cerebrospinal fluid pressure, and nonvasculitic autoimmune inflammatory meningoencephalitis. Lumbar puncture should also be considered when the manifestation is atypical: for example, a rapid course or an early onset of disease. Cerebrospinal fluid can be analyzed for markers of prion disease (14-3-3 protein) and Alzheimer's disease (tau protein and Aβ42 level). Neuropsychological test results characterize the pattern of cognitive strengths and weaknesses and, in this way, are helpful in diagnosis. Also, in mild cases, they may be helpful in determining whether there is a deficit. Furthermore, these tests may be used in monitoring patients over time.

Third-trimester screening

In the routine, uncomplicated adolescent pregnancy, another series of laboratory studies are performed, including some repeat tests from those obtained in the first trimester. These include a CBC, Rh type, gonorrhea and chlamydia screen, as well as RPR and HIV in high-risk populations. At 24–28 weeks of gestation, most patients are also screened for gestational diabetes mellitus. Most clinicians use a two-step diagnostic method starting with a 50 g 1-h oral glucose tolerance test (OGTT). If the initial screening is positive, a 3-h OGTT follows [72]. Universal screening for group B streptococcus (GBS) by vaginal and rectal culture is recommended at 35–37 weeks of gestation. GBS sepsis remains the leading cause of neonatal sepsis and death in nurseries in the United States, and the advent of universal screening endorsed by the CDC, AAP, and ACOG, along with algorithms for intrapartum antibiotics, has resulted in a 70% decrease in newborn GBS sepsis since the 1990s [73]. Several studies have found, however, that a significant number of cases of early-onset GBS sepsis in newborns have occurred in infants whose mothers screened negative for GBC by culture, highlighting the continued need for rapid and accurate testing methods. On admission to labor and delivery, a CBC and ABO/Rh type/Rh are generally drawn as a precautionary measure in the event of excessive or prolonged bleeding that necessitates transfusion. If other screening labs (STIs, hepatitis, rubella) have not been done previously during the pregnancy, they are done at this time.

31 What serologic laboratory tests are used to assist in the diagnosis of syphilis? How should they be interpreted?

Two general classes of laboratory tests are used: nontreponemal antigen tests and treponemal tests. The nontreponemal tests use extract of normal tissues (i.e., beef cardiolipin) as antigens to measure antibodies formed in the blood. The commonly used nontreponemal tests are the rapid plasma reagin (RPR) and Venereal Disease Research Laboratory (VDRL) tests. Both become positive in the early stages of the primary lesion and are almost always positive by the secondary stage. They decline in later stages of the disease. False-positive results occur in autoimmune illness, malaria, mononucleosis, and pregnancy, among others. The nontreponemal tests, therefore, should be used as a screening test for the more specific treponemal tests. They also may be used to follow response to treatment because they decline with time after successful treatment. The treponemal tests use live or killed Treponema pallidum as antigen to detect treponemal antibody directly. The commonly used treponemal tests are the microhemagglutination test for antibody to T. pallidum (MHA-TP) and the fluorescent treponemal antibody absorption test (FTA-ABS). These tests remain positive even after appropriate treatment.

If a patient has strongly positive serum RPR or VDRL and MHA-TP or FTA-ABS tests and symptoms consistent with neurosyphilis, most experts agree that the patient should undergo a lumbar puncture, if feasible, to look for a positive VDRL and/or MHA-TP in the CSF, which helps to confirm the need for treatment for neurosyphilis. These tests may be falsely negative even in CSF. If clinical suspicion is high enough, most clinicians treat for neurosyphilis. Obtaining documentation of prior treatment for syphilis is important because it can obviously shorten the evaluation considerably.

WHAT ADDITIONAL TESTS SHOULD BE DONE?

A 24-hour urine study for heavy metals and porphyrins was negative. A complete blood count revealed mild leucocytosis (white blood cell count, 13.7) with a normal differential. Liver function tests and electrolytes were normal. The erythrocyte sedimentation rate, rapid plasma reagin, fluorescent antinuclear antibody, rheumatoid factor, angiotensin-converting enzyme, C3, C4, complement B12, fatty acid, glycosylated hemoglobin, T4, thyroid-stimulating hormone, and serum immunoelectrophoresis were normal. Antibodies for HIV, human T-lymphotropic virus 1, antinuclear double-stranded DNA, single-stranded A DNA, single-stranded B DNA, SM antibody, and antineutrophil cytoplasmic antibody measurements were negative. West Nile antibody measurements were not available during this time; Lyme antibody, C. jejuni, and GQ1b were negative. A tuberculin skin test was negative. (See Figure 40-1 for the differential diagnosis and work up of patients with acute active paralysis.)

The cerebrospinal fluid (CSF) on admission showed 300 white cells/mL3 (92% lymphocytes); protein was 426 mg/dL (normal, 20–40 mg/dL), and glucose was normal. Gram stain, cultures, cytology, and cryptococcal antigen were negative. Oligoclonal bands were present, and the IgG index was elevated at 59.2 (normal, <0.70). Three days later CSF had a 143 white blood cell count, a protein of 580 mg/dL, and normal glucose.

The increased number of cells were out of proportion for the diagnosis of GBS,1,2 and for this reason other conditions, such as lymphoma, Lyme, HIV, and other infections, need to be ruled out.