IgM tests for detecting Treponema pallidum infection in newborns: time to establish a reference comparator

LETTER

The prevalence of congenital syphilis has increased by more than 1,000% in the USA since 2012 (1). Establishing the diagnosis of congenital syphilis in a neonate, however, remains challenging. Passive antibody transfer from the mother to the newborn renders testing for treponemal or non-treponemal IgG antibodies difficult to interpret. Thus, treatment is frequently indicated solely based on the presence of syphilis in a mother who received inadequate therapy (2), warranting infant hospitalization and intravenous penicillin for 10 days.

IgM assays detect the presence of antibodies formed early in the course of infection. IgM is too large to cross the placenta, and while IgM tests have historically shown cross-reactivity with other antigens (3), a negative Treponema pallidum IgM in an infant may indicate the absence of infection. Research is needed to evaluate the performance of IgM tests in congenital syphilis.

There are several commercially available IgM T. pallidum assays; however, evaluation of those assays (Table 1) was done without standardizing the comparator or stage of syphilis. Immunoblot assays detect antibodies against proteins of different molecular weight, and are between 63.3% and 95.7% sensitive for syphilis in adults. One small study reported a 100% sensitivity (17/17) and negative predictive value (39/39) for congenital syphilis with neurologic involvement using an IgM immunoblot assay against seven T. pallidum serum antigens (4). Enzyme-linked immunosorbent assays (ELISAs) detect antibodies via antibody-antigen binding, and are between 59.8% and 95.7% sensitive for syphilis in adults. IgM fluorescent treponemal antibody-absorption (FTA-Abs) tests studied in the 1970s were between 64.0% and 93.3% sensitive for detecting congenital syphilis (5, 6); however, performance data are not available for current commercially available FTA-Abs IgM assays. Further work is needed to characterize the performance of those IgM tests in cases of congenital syphilis.

TABLE 1.

Commercially available immunoblot and ELISA IgM assays for detecting Treponema pallidum^a

Test type	Manufacturer	Antibodies detected	Target	Performance of IgM component		Comparator	Reference
				Sensitivity	Specificity
Immunoblot
ViraBlot IgG/IgM	ViraMed	IgG and IgM	4 Lipoprotein antigens (47 kD, 44.5 kD, 17 kD, 15 kD) 5 VDRL antigens (1–5)	94.4% (84/89)	99.2% (245/247)b	19S FTA-ABS (IgM)	Manufacturer IFU
BLOT-LINE	Test Line Clinical Diagnostics	IgG and IgM	4 Lipoprotein antigens (47 kD, 41 kD, 17 kD, and 15 kD)	95.7%c	95.2%c	Not reported	Manufacturer IFU
Euroline-WB IgM	Euroimmun	IgM	4 Lipoprotein antigens [47 kD, 45 kD (TmpA), 17 kD, 15 kD]	90.0% (45/50)d	97.1% (266/274)d	Not reported	Manufacturer IFU
recomLine	Mikrogen	IgM	6 Lipoprotein antigens [47 kD, 45 kD (TmpA), 17 kD, 15 kD, Tp257 (Gpd), and Tp453]	63.3 (76/120)	96.7 (29/30)	RPR, TPA, TP-PA, and/or PCR against polA	(7)
LINE IgM	ViroTech	IgM	4 Lipoprotein antigens [47 kD, 45 kD (TmpA), 17 kD, 15 kD]	87.4% (83/95)	98.6% (356/361)	19S FTA-ABS (IgM)	Manufacturer IFU
ELISA
recomWell	Mikrogen	IgM	6 Lipoprotein antigens [47 kD, 45 kD (TmpA), 17 kD, 15 kD, Tp257 (Gpd), and Tp453]	75.0% (90/120)	96.7% (29/30)	RPR, TPA, TP-PA, and/or PCR against polA	(7)
EIA T. Pallidum IgM	Test Line Clinical Diagnostics	IgM	4 Lipoprotein antigens (47 kD, 41 kD, 17 kD, 15 kD)	95.7%c	95.2%c	Not reported	Manufacturer IFU
Euroimmun	Euroimmun	IgM	4 Lipoprotein antigens [47 kD, 45 kD (TmpA), 17 kD, 15 kD]	59.8% (70/117)	100% (30/30)	RPR, TPA, TPPA, and/or PCR against polA	(7)

^a Abbreviations: VDRL, venereal disease research laboratory; kD, kilodaltons (molecular mass of antigen); FTA-ABS, fluorescent treponema antibody absorption test; RPR, rapid plasma reagin; TPA, Treponema pallidum antibody; TP-PA, Treponema pallidum particle agglutination; PCR, polymerase chain reaction; ELISA, enzyme-linked immunosorbent assay; IFU, instructions for use.

^b Two were indeterminant; no false positive.

^c Sample size not reported.

^d Borderline results considered positive (n = 12 of positive specimens; n = 6 of negatives specimens).

The European Union and the British Association for Sexual Health and HIV guidelines recommend T. pallidum IgM testing among newborns suspected of having congenital syphilis (8, 9), contrary to the US Centers for Disease Control and Prevention (2, 10). Several tests are Conformité Européenne marked in Europe and Therapeutic Goods Administration approved in Australia, but no US Food and Drug Administration (FDA)-approved test exists. In fact, there is no reference standard by which a new IgM assay could be assessed. Current FDA serologic reference standards for T. pallidum serologic tests are treponemal IgG or combination IgG/IgM assays (11).

To support the evaluation of T. pallidum IgM tests and test manufacturers’ pursuit of FDA approval, a reference standard must be established. The FDA developed the patient-infected status algorithm to address situations where gold-standard reference tests are unavailable, using a composite standard (12). We recommend the adoption of a composite IgM reference standard based on three test results including two immunoblots and one ELISA or one immunoblot and two ELISAs. At least two positive tests would constitute a positive reference comparator while at least two negative tests would constitute a negative comparator.

IgM assays are needed to address the recent rise in congenital syphilis. Such tests are commercially available and have shown promising performance among adults. However, to support the development and evaluation of such tests, a reference standard in infants must be established.