Performance of solid-state 48 detector gamma counter using solid phase total thyroxin and free thyroxin assays

We evaluated the performance of an automated 48 detector KinetiCount gamma counter using solid phase reagents for total thyroxin (TT₄) and free thyroxin (FT₄). Interassay coefficients of variations (CV) for both total T₄ and free T₄ ranged from 5.4% to 13%. Between-method correlation coefficients were 0.9798 for TT₄ and 0.8844 for FT₄. Detection limits for TT₄ and FT₄ were 3 μg/l and 3 ng/l, respectively. TT₄ was linear up to 200 μg/l. The recovery of TT₄ was between 99 to 101%. A computer-stored standard curve for total T₄ was stable over a 6-day period. The 48 counting chambers demonstrated stability over a 45-day period with no significant changes in counting efficiency (p > 0.05). Precision between the 48 chambers gave a CV of 1.6%. Crosstalk between chambers was <0.3% with count rates up to 80,000 cpm. We conclude that the KinetiCount immunoassay system using solid phase separation technology performs well and is suitable for routine clinical use.