Why two laboratories give different numbers

The method is part of the measurement

A laboratory result is not a universal constant like body temperature — it is the output of a specific instrument, running a specific method, with a specific calibration. Because laboratories often use different testing methods, they often use different reference ranges to describe normal results, and the same sample can legitimately produce somewhat different numbers in different places.

This is not sloppiness; it is how laboratory medicine works. Each laboratory validates its own method against its own healthy population and prints the resulting range on the report.

The comparison mistake everyone makes

The natural move — lining up two reports and comparing the numbers directly — is exactly the wrong one when the reports come from different laboratories. A creatinine of 90 at one laboratory and 100 at another may represent identical kidney function measured by different methods.

The valid comparison runs through the ranges: value versus its own printed range on each report. Where in the range did the result sit then, and where does it sit now? That relative position travels between laboratories far better than the raw number.

How to keep a trend usable

The cleanest strategy for anything that needs tracking — cholesterol, HbA1c, ferritin — is one laboratory over time, where the method stays fixed and the numbers stay comparable. When a switch is unavoidable, the first result at the new place is a fresh baseline rather than a continuation.

Whatever the history, each new report stands on its own range first. And if a discrepancy between two recent reports looks clinically important, the provider may simply repeat the measurement — a cheap way to turn two disagreeing numbers into a trustworthy one.

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