Interference of specific autoantibodies in the assessment of serum thyroglobulin

Prevalence of anti-thyroglobulin autoantibody (anti-TG Ab) above 120 IU/mL was 0.238 in 491 healthy and thyroid-disorder patients (80% female, mean overall age 50 +/- 15 years) presenting to the laboratory for contrastographic, endocrinologic, or general medical purposes. It is well known that anti-TG Ab (autoantibodies to thyroglobulin) can lead to the underestimation of serum thyroglobulin (TG). In this work, recovery (REC) of TG (thyroglobulin) added to the specimen was performed in all TG assays and low REC values were considered indicative of interference. The results show that REC = 70% is a very good cut-off value both for the anti-TG Ab prevalence found in our patients and in all prevalences below 0.6. However, we used an 80% cut-off which shows higher (+11%) sensitivity and in consequence improves the clinical decision. Only 60% of low REC values are directly attributable to high levels of anti-TG Ab. In the remaining 40%, low REC values are associated with low anti-TG Ab (< 120 IU/mL). These data indicate that anti-TG Ab concentrations are not always sufficient for evaluation of interference levels since they do not take account the actual effectiveness of anti-TG Ab. In fact, the same level of anti-TG Ab may reduce REC to different degrees (up to ten times) depending on the source serum. The practical consequence of this marked variability of autoantibody activity is that in the usual double Ab immunoassays REC must be determined for each sample to avoid unsuspected interference and negative influences on the clinical decision.(ABSTRACT TRUNCATED AT 250 WORDS)