Differences between measurements of T4 and T3 in pregnant and nonpregnant women using isotope dilution tandem mass spectrometry and immunoassays: are there clinical implications?
BACKGROUND
It has been established that triiodothyronine (T3) and thyroxine (T4) measurements by tandem mass spectrometry (MS/MS) are more specific and are significantly different from immunoassay (IA) measurements (all p< or =0.05) throughout pregnancy. In this study, we examined the clinical implications of these discrepancies.


METHODS
Kappa statistics were used to determine the degree to which IA and MS/MS agreed in their identification of out-of-reference interval observations of circulating T4 and T3 from 52 normal, iodine-sufficient women during each trimester of pregnancy.


RESULTS
After taking chance agreement into account, the two methods had poor agreement on classification of T3 values at the first (kappa=0.185) and second (kappa=0.183) trimesters, with extremely poor agreement for the third trimester and 1-year postpartum. Agreement on T4 was poor only for the third trimester (kappa=0.183). The two methods agreed on out-of-range values in only 0-25% of T3 cases and 25-66.7% of T4 cases.


CONCLUSIONS
The areas of disagreement suggest that women at risk (i.e., with analyte values outside of the 5-95% range) will not be detected using IA. Based on this cohort, our preliminary estimates are that 25-100% of such women would be missed if IA were used to assay the analyte.
Soldin
Offie P
OP
Division of Cancer Genetics and Epidemiology, Lombardi Cancer Center, Georgetown University School of Medicine, Room S-165A, 3800 Reservoir Road, NW, Washington, DC 20007-2197, USA. os35@georgetown.edu
Tractenberg
Rochelle E
RE
Soldin
Steven J
SJ
D000328:Adult; D002851:Chromatography, High Pressure Liquid; D005260:Female; D006801:Humans; D007118:Immunoassay; D007202:Indicators and Reagents; D013058:Mass Spectrometry; D049590:Postpartum Period / Q000378:metabolism; D011247:Pregnancy / Q000378:metabolism*; D011785:Quality Assurance, Health Care; D011865:Radioisotope Dilution Technique; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*
D007202:Indicators and Reagents; D014284:Triiodothyronine; D013974:Thyroxine