A fast preliminary test for acetaminophen in serum.

48075, in which the 2nd International Standard for HCG is used. This colorimetric method is ideally suited for the small laboratory because of its simplicity and minimal equipment requirements. Essentially, the test is a solid-phase IEM involving pelystyrene tubes coated with anti-HCG (monoclonal mouse IgG). The serum HCG is “sandwiched” (1) by the solid-phase HCG enzymelabeled antibody at room temperature and the tubes are rinsed to remove unbound enzyme-labeled antibody. Enzyme substrate and chromogen are added, and a blue color is produced by the enzyme-linked antibody “sandwiching” the HCG. This blue color absorbs maximally at 370 nni, and is discernible spectrophotometrically for HCG concentrations as low as 5.0 milli-int. units/mL. Because of the proprietary nature of this test, we can only describe the reagents as indicated on the package insert in the kit: pelystyrene tubes (12 x 75 mm) coated with anti-HCG (monoclonal mouse IgG), enzyme-conjugate horseradish peroxidase (HRP) corjugated with anti-HCG, color reagent A (hydrogen peroxide), color reagent B (chromogen), and a wash concentrate that must be diluted 10-fold with distilled water before use. We slightly modified the procedure for the qualitative method, sothat simple observation of the color was replaced by spectrophotometry. This only involved diluting the final reaction mixture with enough distilled water (2 mL) to provide a sufficient volume for measurement. Essentially, a series of 2nd International Standard calibrators ranging from 0 to 100 milli-int. units of HCG per milliliter were processed, as were two controls (0 and 25 milli-int. units HCG per milliliter) and 17 sera from known pregnant and nonpregnant women. Each patient’s serum was testedundiluted and diluted 100-fold. The patients we selected were known to have lower concentrations of HCG, consistent with early pregnancy, or undetectable concentrations of HCG as determined by the RIA method. Such samples test the sensitivity of the modification. We also wished to see whether samples with high HCG concentrations could be accurately estimated. Several samples with HCG values exceeding 20 int. units per milliliter, including one with a value of 65 mt. units per milliliter, as determined by the RIA method were compared with the IEM modifIcation (patients’ sera were diluted1000-fold for this group). Results by the two methods correlated well (as they did also for the high-concentration samples), summarized as follows: n = 17; .‘flIEM) = 1326; 51(RIA) = 964; regression lines: y = 121.5 + 0.63x and x -65.2 + l.44y; SD(x) = 1867; SD(y) = 1233; CV(x) = 140%; CV(y) = 128%; correlation coefficient (r) = 0.95; coefficient of determination (r2) = 0.9; t-test = -1.95. The CVs were calculated by dividing the SD for the 17 results by their mean.