Question 1 · Hard
At 11:00, the routine refractometer check gives: deionized water 1.003; 5% sodium chloride 1.025 (expected 1.022 ± 0.001); 9% sucrose 1.037 (expected 1.034 ± 0.001). The 07:00 check was in range, and 18 patient urines have been read on this instrument since then, one of them at 1.038 with negative glucose and protein and no recent imaging. The prism and cover were cleaned and dried before the check, the room is at 23 °C, and the refractometer is temperature-compensated from 15 °C to 38 °C. What should the technologist do?
- ARelease the results, as the error is trivial
- BSubtract 0.003 from each result and release them
- CRezero with water, repeat QC, then retest the urines
- DCorrect each reading for the room temperature
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Answer: C · Rezero with water, repeat QC, then retest the urines
Every reading, including water, which should read 1.000, is 0.003 too high, so the error is a constant offset across the scale: a zero error. It is corrected by adjusting the zero set screw with deionized water and then rechecking the 5% NaCl and 9% sucrose controls. Because both controls failed, the urines read since the last acceptable check must be retested once QC is back in range. Patient results are never adjusted by hand to make up for a failed control. A reading of 1.038, above the upper reference value of 1.035, with no glucose, protein or contrast medium to explain it, was itself a clue to the problem.
- A: Both controls are outside their ±0.001 limits, so results since the last good check cannot be released.
- B: Results are not corrected by hand; the instrument is fixed and the specimens are retested under acceptable QC.
- D: The refractometer is temperature-compensated across this range, so no temperature correction applies.