Analytical Bias in a Quality Control Scheme - Clinical Chemistry

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1040 ALLEN fT AL Clinical Chemistry Methods The presently used quality control method was instituted 2 years ago with assistance from The Ohio State I. lilversity Statistics l)ivisioii (2-4). Control samples analyzed in the laboratory are of three types. There are samples the technologist knows are controls: These are of pooled serum processed and stored frozen. After the test has beeii standardized, only the first daily result of the 1)001 for every constituent is used for statistical analysis. Tilell there are coniinerical control samples, used at the discretion of the technologist; these results are not kept on record. Finally, there are control samples unknown to the technologist, consisting of commercial control samples at two or three levels of concentration. Oniy the results of the control having a concentration approximating that of the pool have been tabulated (Tables land 2). Control samples, not known as controls to tile technologist, were submitted at randoni times during the work day with a sillluiated patient ielentilication. The time was determined by a table of ratidom numbers. Results of the pool as well as the unknown commercial controls were plotted daily on conlputer-prepared control charts indicating the mean and tile permissible deviation. Values outside the prescribed limits were brought to the attention of the supervisor. The limits used to evaluate daily performance were usually the 2 standard deviations calculated over a 3-month period. The number of analyses is about fit) for each test. No changes iii methodology occurred during this -niontii period for any one test. Results Results of control analysis over a period of 6 months showed differences between sonic of tile known and unknown controls (Tal)le 1). rflle mean values of the known calcium pool and tile blind control for calcium, for instance,differ by about 1% while the standard deviation differsby a factor of about 2. 1xanumation of time results of 18 tests in Table 1 shows that the known control samples for 16 of the 18 tests had a smaller standard deviation and 15 of tile 18 tests had a smaller coefficient of variation than the unknown controls. The prohalility of this occurring by cilance alone is less than 0.001 for the standard deviation and less than 0.01 for the coefficient of variation in a one-tailed test. Since the data did not follow a parametric or normal distribution, a nonparametric statistical analysis, that is, a binomial distribution, was employed for the calculations.
Vol. 15, No. II. 1969 ANALYTICAL BIAS 1041 Table 1. ItEsuhis wirti BLIND (\ERsroI.) ANn KNOWN (Pool.) C sraoi.s .r Onio STATE UNIVERSITY HOSPITALS, 0’T. 1967-MARcH 1968 Specimen .Wean Versatol SI) CV .Vo. Mean Pool SD CV No. Albumin 3.4 0.16 2.9 127 4.3 0.13 3.0 133 Calcium 4.8 0.10 40 127 4.2 0.09 2.2 164 Chb)ridet 102 1 .6 1 .6 127 104 0.8 0.8 182 Creatituiuuet 1.7 0.11 6.3 123 1.9 0.08 4.4 183 (iIobiJin 2.2 0.12 5.7 129 2.6 0.13 5.6 137 (;lucoset 82 :1.4 4.2 126 116 2.7 2.3 183 r.P. 4.3 0.30 6.7 125 4.0 0.21) 4.9 126 Kt 3.0 0.13 3.0 126 4.7 0.12 2.5 183 Nat 140 2.4 1.7 127 143 1.2 0.8 l83 Tot, protein 7.6 0.19 2.5 12$ 73) 0.17 2.4 136 Urea-Nt 12.5 0.8 6.6 125 25.2 0.8 3.0 183 Uric Acidt 3.9 0.27 4.5 173 5.6 0.22 4.0 182 Cholesterol 224 13 6.5 127 200 7.0 :1.3 124 Alk. p’tase 6.9 0.4:1 6.2 126 2.6 0.15 3.6 127 Amylase 230 17 6.9 127 103 10 9.7 173 LDH 303 52 10.4 12(1 261 24 9.3 131 SOOT 133 12 9.3 121 23.7 3.2 13 129 SGPT 10.7 3.0 28 111 18.9 3.1 16 121 * Ntunber of analyses for each constituent is equal to the number of days on which the given procedure ha.s beemi reported. f Automated procedures. Examination of each of the seven automated tests shows that the average of the monthly standard deviations and of the monthly coefficients of variation of the unknown control samples was greater than the known samples in all cases (7+). The probability of this occurring by chance is less than 0.01. Of the 11 nonautomated tests, tile standard deviation of tile unknown controls was greater than that of known controls in 9 instances, and tue coefficient of variation was greater in eight tests. The probability of this occurring by chance is 0.027 for the standard deviation arid 0.081 for the coefficient of variation. Discussion The data in Table 1 illustrate tile variation between “known” controls and ‘‘blind’’ controls in several routine clinical chemistry tests. The “blind” sample, more like that of a patient’s specimen, measures Ilie precision actually achieved Ill the laboratory, in contrast to “known’’ controls which measure tile precision the laboratory is capable of achieving. It should 1)e emphasized that these data were collected by doing just one set of control samples a day, and except for the known control samples chosen by the technologist for checking
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Analytical Bias in a Quality Control Scheme - Clinical Chemistry

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