LAB NEWS
March 1995 . . . . . . . . . . Vol. 37 No. 1

DIAGNOSIS OF CONGENITAL SYPHILIS

Serological tests for syphilis include both nontreponemal tests (VDRL, RPR) and treponemal specific assays (FTA-ABS, MHA-TP, IgG and IgM anti-T.pallidum antibodies). There are three potential sources of serological information for assessment of the newborn: maternal serum, umbilical cord blood serum, and neonatal serum. Of these three sources, cord blood serology remains the most problematic and is of limited usefulness in clinical management.

Use of Cord Blood Samples
Cord blood is easily contaminated with Wharton's jelly, which causes false positive nontreponemal test results. This is especially likely if the proper procedure for collecting cord blood is not followed. This procedure involves cleaning the outside of the umbilicus, double-clamping it, and making a fresh cut. While good procedure will help to eliminate false positives, the analysis of cord blood is also fraught with a major problem of false negative results. In a recent survey of over 3000 deliveries at the Bronx Municipal Hospital in New York, cord blood serologies missed between 10 and 25 percent of babies with presumptive congenital syphilis when compared with the use of neonatal serum obtained on the babies two to three days after delivery and when compared to the use of maternal serum. Thus, although venerable in practice because of its relative ease, cord blood serology is the least efficacious diagnostic approach to congenital syphilis.

Use of Maternal Serum Samples
All studies have also confirmed that testing of maternal serum is superior to both newborn and cord blood serologies in screening for congenital syphilis because of "false negatives" on newborn serum. The causes of negative neonatal serology in the presence of a positive maternal serology include: extreme prematurity with inadequate placental transfer of maternal antibody, infection in the infant acquired in pregnancy near term before the production of antitreponemal antibodies, and the disappearance of antibodies associated with concurrent HIV infection. It should be noted that another cause of false negatives - the prozone phenomenon - should not be a significant issue at YNHH because the Immunology Laboratory screens against such an occurrence.

The optimal approach to preventing congenital syphilis is serial screening of the mother during the prenatal period, preferably at the first prenatal visit, during the third trimester, and at the time of delivery. Unfortunately, this level of prenatal care does not occur for a significant number of women and often only testing at the time of delivery is available. The CDC changed its case definition for congenital syphilis in 1989 so as to include, as a presumptive case of congenital syphilis, any infant whose mother had untreated or inadequately treated syphilis at the time of delivery. On this basis, one could argue for treatment of all infants whose mothers at delivery had a positive VDRL, confirmed with a positive FTA-ABS. However, since up to 40% of these neonates are not infected, a test to distinguish infected versus uninfected babies remains an important priority.

Because of transplacental transfer of maternal IgG, neonatal VDRL's and FTA's can easily be positive in the absence of infection and use of newborn versus maternal comparative IgG titers has not proven to be of consistent value. By contrast, since IgM is a large pentameric molecule that does not cross the placenta, the presence of specific IgM antibody in a newborn implies that the baby has indeed directly encountered the antigen of interest. The CDC has, therefore, compared several alternative assays based on measurement of neonatal IgM, including the FTA-ABS 19S IgM assay, the IgM capture ELISA assay, the reverse enzyme-linked immunospot (relispot) assay and a Western blot test. The FTA-ABS IgM test and relispot have not given consistent results in clinical applications. The Western blot assay remains investigational and is not recommended by CDC for clinical use at this time. The capture ELISA assay, however, has now obtained "provisional" status, meaning that the CDC feels the test can be used to help in the clinical management of patients but only in the context of other test results and not as the sole criterion for diagnosis.

The CDC-recommended standard protocol for monitoring newborns is as follows: Mothers should have a VDRL or RPR performed near delivery with a confirmatory FTA-ABS if the VDRL/RPR is positive. For mothers who have a positive VDRL or RPR at the time of delivery and/or a history of syphilis which is untreated, or for whom treatment history cannot be documented, the infant should receive an evaluation for clinical signs and symptoms of syphilis.

This usually includes a thorough physical examination for evidence of congenital syphilis and a quantitative nontreponemal serological test (VDRL or RPR) performed on infant serum (NOT cord blood). It often also includes a lumbar puncture for cerebrospinal fluid analysis of cells, protein, VDRL/CSF (note that only a VDRL can be used on CSF as a standard test for syphilis), long bone X-rays, and any other tests clinically indicated by the patient's status.

If any signs or symptoms of congenital syphilis are found, then the diagnosis is made, regardless of baby's IgM antibody status. If there are no signs of congenital syphilis, then the determination of the presence of T.pallidum-specific IgM antibody by the capture ELISA can be made and interpreted in the context of the neonate's clinical state as outlined in Table 1. The appropriate specimen for this IgM test is newborn serum. The presence of IgM anti-T. pallidum is considered suggestive of congenital syphilis requiring therapy even for babies with no clinical signs or symptoms. Note that a negative IgM serology in the neonate is not sufficient to rule out infection since some babies may show clinical evidence of syphilis but have a negative IgM study.

To order the IgM capture ELISA assay, an immunology or general laboratory slip should be used. "IgM T.pallidum Ab" or "syphilis IgM" should be written on the slip and sent with newborn serum (NOT cord blood) to the Immunology Laboratory. If there has not been a VDRL previously performed on newborn serum, this will be added by the laboratory. It should be noted that this IgM test has so far not proven useful in diagnosing adult patients with recurrent syphilis and hence should not be used for that purpose.

Because of recent trends in healthcare emphasizing rapid discharge of patients after delivery, the steady incidence of women delivering without prenatal care and the fact that these women are often easily lost to follow-up after delivery, a need has developed to occasionally obtain screening maternal syphilis serologies on weekends. The procedure for doing so is to call the Laboratory Medicine resident and briefly describe the clinical situation and anticipated time of discharge. The Laboratory Medicine resident will explain the procedure for sending the sample under these circumstances. Generally, an RPR rather than a VDRL will be performed. Note that this will only be carried out on maternal serum or newborn serum, but not on cord blood, consistent with CDC recommendations. The IgM assay discussed above cannot be performed on a stat basis.

Finally, it is important to note that syphilis serological testing is in a state of continuous flux. Active work by the CDC for defining newer antibody tests either by ELISA or Western blot are underway. Such tests will be incorporated in the YNHH Clinical Laboratory if and when they become approved by the CDC for clinical use.

References

1. Centers for Disease Control. Sexually Transmitted Diseases Treatment Guidelines. MMWR. 1993; 42 (RR-14):40-41.
2. Chabrars, Brion LP, Castro M at el. Comparison of Maternal Sera, Cord Blood, and Neonatal Sera for Detecting Presumptive Congenital Syphilis. Pediatrics. 1993; 91: 88-91.

Brian R. Smith, M.D.

Table 1

TABLE 1 Interpretation of newborm IgM syphilis serologies for the diagnosis of congenital syphilis
Infant's IgM syphilis result	Clincal signs and symptoms in infant	Mother's treatment history	Dianosis
Nonreactive	Present	Untreated or inadequately treated	Congenital syphilis
Nonreactive	Absent	Untreated or inadequately treated early syphilis	Possible incubating congenital syphilis
Nonreactive	Absent	Untreated or inadequately treated latent syphilis	Unknown risk of congenital syphilis
Reactive	Present	Untreated or inadequately treated	Congenital syphilis
Reactive	Absent	Untreated or inadequately treated	Suggestive of congenital syphilis