Estimation of true serum thyroglobulin (Tg) value using simultaneous measurement of serum antithyroglobulin antibody(TgAb)

Objectives Although Tg is the most sensitive marker for detecting recurrence of differentiated thyroid cancer(DTC), presence of TgAb interferes Tg measurement and makes the observed Tg value lower than the true Tg value. Influence of TgAb on the measured value of Tg is known to be positively correlated with the concentration of TgAb but still there is no estimating method to adjust measured Tg value to true Tg value accords to concentration of TgAb yet. In this study, authors tried to develop a mathematical equation which estimates true Tg value under various concentrations of TgAb using both standard solutions of Tg and TgAb measurement kits and patients' serums having high Tg or high TgAb.

Methods Serum Tg was measured using immunoradiometric assay kit (Tg-plus, Brahms) and TgAb was measured using an automated radioummunoassy kit (anti-Tgn, Brahms). Tg measurement was preformed with mixture of Tg and TgAb standard solutions. The measurement also performed with mixture of high Tg serum and high TgAb serum. Expected Tg values of the mixtures were calculated and compared with observed Tg value for the mixtures.

Results Observed Tg values are highly reproducible in both mixtures (p<0.001). Observed Tg values of mixtures having TgAb was significantly lower than their expected Tg value (p<0.001). Magnitude of TgAb interference with the Tg assay was positively correlated with concentration of TgAb. Expected Tg value was successfully calculated with following equation using concentration of TgAb and observed Tg value (Expected Tg (ng/mL)= 0.46+0.35(Observed Tg (ng/mL) * SQRT TgAb (U/mL)) or =-1.24)+0.07(Observed Tg * SQRT TgAb), R2=0.99, p<0.001).

Conclusions Authors has produced a mathematic equation predicting true Tg value using observed Tg value and concentration of TgAb. The estimated true Tg value by the equation might be more valuable than observed Tg value for predicting presence of residual or recurrent cancer lesion in DTC patients

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Influence of TgAb to Tg measurement