Dr. Aliyah Vargas had run the University Hospital’s clinical chemistry lab for twelve years, and in that time, she had learned to trust two things: cold logic and the CLSI guidelines. EP28, specifically—the standard for defining, establishing, and verifying reference intervals—was her bible. It told her what “normal” looked like for a patient population.
She called Mrs. Park’s family. The levothyroxine was stopped. The arrhythmia resolved. clsi ep28
So when the new automated immunoassay analyzer arrived, she knew the drill. The manufacturer’s reference intervals for thyroid-stimulating hormone (TSH) were neatly printed in the manual: 0.4–4.0 mIU/L. But EP28 was clear: Verify before use. Don’t trust, verify. It told her what “normal” looked like for
She pulled the raw data from her 120 healthy subjects. Most were young—residents, techs, nurses under 40. Only seven were over 65. The elderly subgroup, small as it was, had a higher median TSH. The levothyroxine was stopped
The root cause analysis landed on Aliyah’s desk. She stared at the EP28 document, the same dog-eared copy she’d used for twenty years. And then she read the section she’d always skimmed:
And Aliyah learned that “normal” is not a number printed in a manual or even a percentiles from a tidy dataset. It is a fragile, shifting border between biology and statistics—and the job of a clinical chemist is not just to measure, but to interpret who, exactly, is in the room when you draw the line.
