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IgM rheumatoid factor removal and performance of the FTA-ABS (IgM) test in congenital syphilis
1Department of Paediatrics and Child Health, University of Cape Town, South Africa.
Objective:
To determine the performance of the FTA-ABS (IgM) test in congenital syphilis after eliminating interference by IgM rheumatoid factor (RF) and preventing competitive inhibition by IgG.
Design:
The FTA-ABS (IgM) test was carried out before and after RF removal (achieved by immunoprecipitation of the IgG) in infants with congenital syphilis and controls.
Setting:
Newborns delivered in the Peninsula Maternal and Neonatal Services in Cape Town and infants presenting at Red Cross War Memorial Children's Hospital.
Subjects:
Infants with congenital syphilis aged 0-4 months were divided into those with clinical signs at presentation and those who were asymptomatic at delivery. In addition, patients without congenital syphilis but with similar clinical signs at presentation were investigated as were control infants.
Outcome Measure:
The diagnosis of congenital syphilis was based on the criteria suggested by Kaufman et al (1977).
Results:
Amongst symptomatic infants with congenital syphilis the FTA-ABS (IgM) test was positive in 34 (92%) of 37 cases prior to abolishing the RF effect and in 29 (78.4%) of 37 cases afterwards (p = 0.19). In 12 cases of congenital syphilis who were asymptomatic at birth, 10 had positive FTA-ABS (IgM) tests before RF removal and only three had positive tests afterwards (p = 0.006). False positive tests were not found amongst 15 symptomatic infants whose clinical features mimicked those of the infants with congenital syphilis. Among 51 healthy infants the test had a false-positive rate of 2% in newborns and 13% in older infants. The false positive reactions were eradicated by IgG precipitation.
Conclusions:
Following IgG and RF removal there was an improvement in the specificity of the FTA-ABS (IgM) test but this was at the expense of a loss of sensitivity, particularly in asymptomatic newborns. For newborns, if the FTA-ABS (IgM) test was positive, the patient was likely to require treatment for congenital syphilis, regardless of whether the result was due to the presence of RF or specific IgM.