Scottish Immunoassay Support Service Quality Control Scheme for Thyroxine, Triiodothyronine, and Digoxin Assays: Analysis of First 18 Months' Experience

Abstract

MethodsSince the survey of Bold and Browning (1975), methods for protein-bound iodine have largely been replaced by competitive protein binding (CPB) and, latterly, by radioimmunoassay (RIA) methods for T4. This report analyses the performance of current thyroid function tests and digoxin assays in routine clinical practice over an 18-month period. Biochemistry, 1978, 15, 203-207 For T4, radioimmunoassay (RIA) methods gave values close to the all-method mean. Competitive protein binding (CPB) kits gave values significantly higher (by 12~{;) in one and lower (by 10 %) in another, probably due to the extraction of interfering substances such as non-esterified fatty acids or thyroid hormone binding proteins. The range of between-batch precision of individual laboratories was 10,8-47,5~{; (mean 17·8 %) over the clinically relevant range. It was particularly poor at levels below the lower limit of the normal range. Annals of ClinicalFor T3 RIA, kit methods gave significantly higher values than independent methods although recovery of exogenous T3 was approximately quantitative with both. Cross reaction of T4 in T3 assays was insignificant. The range of between-batch precision for individual laboratories was 11·4-35·5 % (mean 21·9 %), and was poorest at levels of I nmol/I or less.For digoxin RIA, Lanoxitest y gave significantly higher values than other methods, and overrecovered exogenous digoxin. The range of between-batch precision was 13·8-35·2% (mean 21·6%), and was poorest at levels of 1 nmoljl or less.The distribution of liquid human serum specimens at ambient temperature was satisfactory for quality control of T4, T3, and digoxin RIA but not for T4 by the Thyopac 4 method. Ox serum was unsuitable for certain T4 RIA methods.proteins and were Australia antigen negative. Sera were stored at -20°until despatched by first-class mail at ambient temperature. Concentrations of T4, T3, and digoxin were stable for the period of use of an individual pool (up to eight months) when stored at -20°. Laboratories were advised to store specimens frozen or at 4°until assay.In recovery experiments, L-T3 (free acid), L-T4 (sodium salt pentahydrate), and digoxin, aU from Sigma Chemical Co, and L-T4 (free acid, T3 0·1 %)The Inter-Area Quality Control Scheme started in from Henning Berlin GmBH, were dissolved in a September 1975as part of the Scottish Immunoassay small volume of ethanol before addition to serum Support Service. Between three and six coded pools. T4-, T3-, and digoxin-free sera and plasma specimens were circulated every two weeks from a were prepared by treatment with charcoal, according total of 17 pools, each pool being circulated from to the method of Larsen (1972). five to 12 times each. Nine pools consisted of human Results greater than 3 SD from the method mean serum made available by the Blood Transfusion were excluded from calculation of method means Service, seven pools consisted of reconstituted dried and recoveries but were included in the assessment ACD plasma, and one pool was ox serum. These of preci...

Enzyme Immunoassay: A Review

“…In practice, the precision of many EIAs appears to be comparable to that of other immunoassay systems, and the results obtained seem, in most instances, to correlate reasonably well with those obtained from RIA and other systems. However, the situation will be completely clarified only when EIAs are used routinely and the results analysed by quality control schemes, as is the case with a number of RIAs (Ratcliffe et al, 1978).…”