To the Editor.—
I believe that Dr Burdick (234:1019, 1975) erred in his criticism of Dr Barnett's discussion of the 12-factor automated chemical analysis (SMA-12) profile (233:911, 1975). Using 95% confidence limits and screening a presumably healthy population, one would not expect that "most panels will have at least one value outside the normal range." Assuming independence of channels, one can easily calculate the expected percentage of panels with at least one abnormal value: (1—0.9512)× 100% = 46%.However, the assumption of independence may not be valid for the SMA-12 profile. For example, serum glutamic oxaloacetic transaminase, alkaline phosphatase, and bilirubin determinations may reasonably be expected to be interdependent. Interdependencies will tend to reduce the percentage in question. Although an expected value cannot be calculated because mathematical descriptions of the interdependencies are not available, the published data of Cunnick et al1 are relevant. In their analysis of 1,000
Marciniak TA. Reliability of SMA-12 Profile. JAMA. 1976;235(21):2285. doi:10.1001/jama.1976.03260470013009
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