685 COMPARATIVE STUDY OF

ANTI-HAV-IgM

ACTIVATION OF RHEUMATOID FACTOR DURING PREGNANCY

DETECTION

SIR,—The IgM-class rheumatoid factor (R.F.) can cause false-positive results when IgM antibodies in human serum are detected by labelled anti-human-IgM immunoglobulins in, for instance, immunofluorescence or radioimmunoassay (R.I.A.) tests.’2 This is a serious problem, especially when pregnant women are

positive

examined for can lead

result

rubella infection, since a falseunnecessary abortion or at least

acute to

the patient anxiety. We have lately seen few such false

cause

positives result in our for rubella IgM antibodies,2 but these patients have almost invariably been pregnant. Since the prevalence of high R.F. titres in a normal population was low3 and since acute rubella infection did not activate R.F. in males,4 we investigated the effect of pregnancy on R.F. production. The R.F. levels of 88 pregnant women (mean age 27, range 17-37) were determined by a solid-phase R.I.A.3 and compared with R.F. levels of 111 female blood-donors (see figure). The median value for blood-donors was 5 R.F. units and for pregKendall’s S test). Included in nant women 35 units (P