Thyroid function testing evaluated on three immunoassay systems

Total thyroxine (TT 4 ), tracer uptake, thyroid‐stimulating hormone (TSH), and free T 4 (FT 4 ) testing was examined in the ACS 180, Abbott IMx, and Stratus II systems. TSH sensitivity studies demonstrated that the ACS and IMx are second‐generation assays; the Stratus showed between first‐ and second‐generation performance. Except for one control below the detection limit, TSH imprecision was 3–10%. TSH accuracy was adequate for all systems. TT 4 imprecision was 4–7%, except for the lowest control with the Stratus (18%). TT 4 method agreement showed bias of about 20% between the systems. TT 4 accuracy was compromised at the low end for the Stratus II, but was satisfactory otherwise. FT 4 imprecision was 20% for the Stratus, <5% for the IMx and 6% for the ACS. Uptake assays showed imprecision of 5–10%. The laboratory diagnosis indicated by each system's FT 4 and calculated FT 4 Index results were compared to determine diagnostic accuracy. In routine specimens, different clinical classifications were observed for 25% of specimens with the ACS, 19% with the Stratus, and 7% with the IMx; in the altered protein group, the Stratus showed 9% discordant results, the ACS showed 3%, and the IMx 0%. Of the three systems examined here, the Abbott IMx system showed the best overall performance.©1995 wiley‐Liss, inc.