← All chapters·Chapter 04 · free opening

Why a single test result means nothing

The within-subject variation of hs-CRP, the change a result has to exceed before it carries information, and the two situations where testing is still worth it.

This chapter was originally meant to explain how to establish a baseline. The data turned it inside out. What it now explains is why the before-and-after test sold across this entire category measures noise — which is a more useful thing to know and saves more money than the publication costs.

The variation

B Across 60 studies of biological variation, the median within-subject coefficient of variation for hs-CRP is 0.44 (range 0.27–0.76). For standard CRP it is 0.41, with intraclass correlation between 0.55 and 0.62 (Gough 2024, PMID 39485740).

A within-subject CV of 0.44 means that in a person whose underlying state has not changed at all, repeat measurements scatter by tens of per cent as a matter of routine.

5 more sections in this chapter

The reference change value

There is no such thing as an inflammation level

Where the test does carry information

What would make a self-test informative

The cost side, stated plainly

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