Direct and indirect parameters of free thyroxine. I. Method, quality control and in vitro experiments on storage of samples and effect of drugs
Reiners, C.; Hoffmann, R.; Moll, E.; Börner, W.
Nuklearmedizin. Nuclear Medicine 22(6): 263-272
1983
ISSN/ISBN: 0029-5566 PMID: 6669473 Document Number: 213057
The clinical applicability of direct FT4 determinations by the radioimmunoassays ImmoPhase, GammaCoat, Liquisol, Amerlex and LisoPhase is tested in comparison to the FT4 index and the T4/TBG ratio. Part I of this study deals with technical aspects concerning quality control as well as with in-vitro experiments concerning storage of samples and interference of drugs. The precision of FT4-RIAs is satisfactory; inter-assay coefficients of variation are generally lower than 10% for medium hormone concentrations. Because of a change of the standard curve calibration by the manufacturer of the Liquisol kit, the inter-assay variability (17%) of this assay is unacceptably high. The comparison of working ranges shows that the sensitivity of all FT4 assays is sufficient. However, the upper limits of working ranges of the Liquisol and LisoPhase kits are too low. Storing the samples in form of serum yields lower values for any FT4 parameter than storing as whole blood. This unexpected observation is true for room temperature as well as for refrigeration at 4 degrees C. To be sure, the values of indirect FT4 parameters from serum and whole blood coincide somewhat better especially when the samples are stored in the refrigerator. Repeated thawing and refreezing does not influence the results of FT4-RIAs significantly. In vitro testing of drug interferences by addition of therapeutic doses of primidone, phenobarbital, phenytoin and acetylsalicylic acid yields significant effects only for the last quoted pharmaceutical. While the FT4 levels of the Liquisol and LisoPhase assays rise with the acetylsalicylic acid dose according to expectations, the values of the Amerlex and GammaCoat assays decrease. With regard to handling of the tests, there are no special technical problems for all FT4-RIAs tested.