The method of bracketing is preferred when the highest measurement accuracy is needed and the instrument response is linear between the two bracketing standards. The bracketing method corrects for signal and baseline drift during the analysis. Bracketing measurements are made in the order: low standard, sample, high standard, sample, low standard, sample, ... etc. The calcium concentrations are calculated using the following formula:

Concentration = Conc Low Std + (Conc High Std - Conc Low Std) *( (Absorbance Sample - Absorbance Low Std)/(Absorbance High Std - Absorbance Low Std))

Based on the method of bracketing (see the Excel 97 spreadsheet for details of the calculation), the patient's serum calcium concentration is 10.34 ± 0.48 mg/dL (95% confidence limits). The normal range for Ca in human serum is 8.5 mg/dL to 11.0 mg/dL, so the concentration falls within normal limits. However the results for the control, SRM 909b, are 7.60 ± 0.08 mg/dL and the certified value for that material is 9.87 mg/dL of Ca with an uncertainty (95% confidence limits) of 0.08 mg/dL. Therefore, the method appears to be biased by approximately 20%, and although the patient's serum calcium falls within the normal limits, it may actually be much higher, depending on the reason for the bias. The method does not meet the precision requirements of 2% for the patient's sample, although it does for the control. Based on the results for the control, the method is not accurate. The patient must be retested and the bias in the method must be corrected.

Explain what mistakes might have caused bias in the method.

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