Interassay variability of immunometric methods for thyrotropin in an external quality assessment survey: evidence that functional sensitivity is not always adequate for clinical decisions.
We investigated the ability of current immunometric methods for thyrotropin (TSH; thyroid-stimulating hormone) to distinguish between low-normal and subnormal hormone concentrations by using the data from an external quality assessment (EQA) survey in 1990. We computed the interassay (between-run) precision profiles from results from 101 laboratories, which used the five most popular kits in the survey; during the control period (one year) each laboratory assayed 4 EQA pools distributed (as hidden replicates) in five occasions. The interassay CV was relatively low (9-13%) for three pools in the normal TSH range (greater than 0.8 milli-int. unit/L) but markedly higher (30-40%, except for one more precise kit) in the subnormal range (0.2 milli-int. unit/L). We calculated the effect of the between-run variability on the diagnostic accuracy (discrimination between normal and subnormal values) for three representative TSH concentrations: 0.2, 0.4, and 0.5 milli-int. unit/L (0.3 milli-int. unit/L was considered the lower normal limit). The three concentrations were reasonably discriminated (P less than or equal to 5%), and only one kit showed a between-run CV less than 18% at 0.2 milli-int. unit/L. For the other four less-precise kits, only the higher TSH value (0.5 milli-int. unit/L) could be classified with an acceptable diagnostic reliability. With the most precise kit, one can distinguish two TSH concentrations in the 0.3-0.5 milli-int. unit/L range that differ by at least 30%; with the other kits, differences greater than 50-60% are needed for reliable discrimination. Thus many laboratories fail to achieve the functional sensitivity of a second-generation assay, even if they use immunometric methods. TSH assays with a better interassay precision in the low concentration range are needed.
Pilo
A
A
CNR Institute of Clinical Physiology, Pisa, Italy.
Zucchelli
G C
GC
Malvano
R
R
Clerico
A
A
Iervasi
G
G
Signorini
C
C
D006801:Humans; D007118:Immunoassay / Q000592:standards* / Q000706:statistics & numerical data; D011786:Quality Control; D011933:Reagent Kits, Diagnostic / Q000592:standards / Q000706:statistics & numerical data; D013972:Thyrotropin / Q000097:blood*
D011933:Reagent Kits, Diagnostic; D013972:Thyrotropin