Ulrich Häring Erwin

FT3 values. FT3 values in the 2 patients with antibodies to T2, T3, or their conjugates were 12.3 and 32.6 pmol/L for the Vitros FT3II assay. With the Vitros FT3 assay, FT3 concentrations in 2-fold diluted control sera were 96% (1.1%) [mean (SD)] of those in the nondiluted sera, and in 2-fold diluted sera from 2 patients taking diclofenac, FT3 concentrations were 95.9% and 96.3% of those in the nondiluted sera (Fig. 1) With the Vitros FT3II assay, FT3 concentrations in 2-fold diluted control sera were 97% (0.7%) of those in the nondiluted sera. For 2-fold diluted sera from 2 patients taking diclofenac, FT3 concentrations were 98% and 97% of those in the nondiluted sera, indicating that with the Vitros FT3 assay the dilution effects on FT3 concentrations did not differ substantially in sera from patients taking diclofenac and control sera. This result suggests that the concentration of free diclofenac in the bloodstream does not change with dilution, because diclofenac is strongly bound to serum proteins and thus obeys mass action dilution criteria. With the Vitros FT3II assay, the FT3 concentrations in sera from 2 patients with antibodies to T2, T3, or their conjugates decreased 71% and 55% with dilution. This result suggests that, in the Vitros FT3II assay, antibodies to T2 or its conjugate interfered with binding of the T2-confugate. It should be noted that the converse phenomenon may occur in the presence of large quantities of weaker binding drugs or lower, nonsaturating quantities of autoantibodies. In conclusion, because the serum concentration of free drug (diclofenac) in serum is not altered by dilution, according to the mass action dilution criteria, the measured FT3 concentration in the serum remains falsely high. When the interfering substance (autoantibody) leading to falsely high FT3 concentrations is diluted, the interference diminishes and measured serum FT3 concentration decreases. This is the first report to show that dilution tests can differentiate between drugs and interfering substances in serum.