Clsi Ep28 -

Aliyah nodded. “But EP28 says if we have 120 subjects, nonparametric ranking is the gold standard. The 2.5th and 97.5th percentiles are 0.6 and 3.2. That’s our truth.”

The conflict tore the lab apart. Clinicians started calling. A healthy medical student with a TSH of 3.8—perfectly fine by the old book—was now flagged high. An exhausted intern with a TSH of 0.5 was flagged low, even though she felt fine after a night shift.

That night, Aliyah wrote a new lab policy. They would adopt the manufacturer’s broader interval for patients over 65—not out of laziness, but out of a deeper respect for EP28’s core principle: A reference interval is only as good as its reference population. clsi ep28

Mrs. Park wasn’t abnormal. Aliyah’s reference population was just too young.

Three weeks later, Mrs. Park was in the ER with atrial fibrillation—a known risk of overtreatment in the elderly. Aliyah nodded

She called Mrs. Park’s family. The levothyroxine was stopped. The arrhythmia resolved.

“Reference intervals may need to be partitioned by age, sex, or other factors… especially for analytes like TSH, where values increase with age.” That’s our truth

Then came the case that changed everything.

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.

The lower limit of her in-house reference interval was 0.6 mIU/L. The upper limit was 3.2.

Mrs. Eleanor Park, 68, came in for fatigue. Her TSH was 3.9 mIU/L—within the manufacturer’s range but above Aliyah’s verified upper limit of 3.2. Using the lab’s new narrow interval, the computer flagged it as Abnormal-High . The junior resident started her on low-dose levothyroxine.