[Effect of heparin lithium as anticoagulant in assay of FT3, FT4 and TSH]

To evaluate the application of heparin lithium as anticoagulant in in vitro tests of thyroid function.

The blood samples obtained by venipuncture from 32 subjects (including 10 normal subjects and 22 patients with thyroid disorder) were collected in parallel dry vacuum tubes with one of them containing heparin lithium, for assay of TSH, FT3, FT4 by chemoluminescence immunoassay.

No difference was found in TSH and FT4 levels determined separately from the parallel tubes (TSH: t=1.846, P=0.075; FT4: t=1.649, P=0.110) with closely correlated results (TSH: r=1.000, P=0.000; FT4: r=0.999, P=0.000), and the clinical diagnoses therefrom derived were perfectly matched. FT3 level in ordinary dry vacuum tubes, however, was significantly higher than that in the tube containing heparin lithium (t=-6.253, P=0.000), but still close correlation was observed between them (r=0.999, P=0.000 0). Inconsistent clinical decisions occurred in 7 of the 32 subjects in respect of FT3 levels respectively assayed in the 2 tubes, but when the normal ranges of FT3 level were established for the 2 tubes separately, the same clinical diagnoses were reached.

TSH and FT4 levels as determined in the 2 tubes are comparable, and even though FT3 levels does not present this feature, close relation is obvious and therefore the 2 values can be equivalent after establishment of their respective normal limits or after linear regression processing.