In a murder trial in the 1990s, the defendant's blood was found at the crime scene. The prosecutor argued that blood was left by the defendant during the crime. The defense argued that police "planted" the defendant's blood from a sample collected later. Blood is normally collected in a vial containing the metal-binding compound EDTA as an anticoagulant with a concentration of ~4.5 mM after the vial has been filled with blood. At the time of the trial, procedures to measure EDTA in blood were not well established. Even though the amount of EDTA found in the crime-scene blood was orders of magnitude below 4.5 mM, the jury acquitted the defendant. This trial motivated the development of a new method to measure EDTA in blood.
(a) Precision and accuracy. To measure accuracy and precision of the method, blood was fortified with EDTA to known levels.

For each of the three spike levels in the table, find the precision and accuracy of the quality control samples.
EDTA measurements (ng/mL) at three fortification levels

(b) Detection and quantitation limits. Low concentrations of EDTA near the detection limit gave the following dimensionless instrument readings: 175, 104, 164, 193, 131, 189, 155, 133, 151, and 176. Ten blanks had a mean reading of 45.0. The slope of the calibration curve is 1.75 × 109 M-1. Estimate the signal and concentration detection limits and the lower limit of quantitation for EDTA.

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mean value found - known value Accuracy = 100 x known value standard deviation coefficient of variation Precision = 100 × mean 88.2 ng/mL 83.6 69.0 83.4 100.0 76.4 Spik: 22.2 ng/mL Found: 33.3 19.5 23.9 20.8 20.8 314 ng/ml 322 305 282 329 276