In one sentence
A protein the liver releases in response to inflammation anywhere in the body, which climbs a hundredfold with an infection and, at the quiet end of its range, needs a high-sensitivity assay to be read at all, because an ordinary assay reports anything under 5 mg/L as "less than 5".
Why it matters
The quiet end is the one that matters for a well adult. In 160,309 people without a history of vascular disease from 54 prospective studies, followed for 1.31 million person-years, each standard deviation of log CRP (a threefold higher concentration) was associated, after adjustment for conventional factors, with risk ratios of 1.37 for coronary heart disease, 1.27 for ischemic stroke, 1.55 for vascular mortality and 1.54 for mortality from causes other than vascular disease, associations of broadly similar size that the authors called unclear in their relevance and that depended considerably on the conventional factors (Emerging Risk Factors Collaboration 2010). CRP is a marker of a process, in other words, not a cause: it reports that something is inflamed, which may be body fat, gums, a lung, a joint or last week's cold. The bands most laboratories print for a high-sensitivity result come from the 2003 statement of the US Centers for Disease Control and the American Heart Association: below 1, 1 to 3, and above 3 mg/L, with a result above 10 read as an acute cause and repeated once it has settled (Pearson 2003).
What moves it
Acutely, for days to weeks: any infection, an injury, surgery, a vaccination, dental work, a hard training session. Chronically: body fat, which releases the signals that drive it, so that it tends to fall with weight loss and with months of regular exercise; smoking; short sleep; gum disease; oral estrogen. Because it moves so readily, one high value is a question, not a finding.
How to measure it well
Ask for high-sensitivity CRP. A standard CRP, which is what most New Zealand community laboratories run unless asked, reports "<5" below its limit, a censored value that cannot be compared with VT's working optimal and cannot feed PhenoAge's log CRP term. No fasting needed. No draw within two weeks of being unwell or of a vaccination, and no hard training for 48 hours. Two results some weeks apart say more than one.
How Vitality Tracker uses it
Stored as reported with its comparator, so "<5" stays "<5": on the chart it is drawn as an open marker at the bound, and against the optimal it is treated as unknown rather than as a number. Typical laboratory reference < 5 mg/L; VT's working optimal (v0.1) < 1.0 mg/L, shown beneath in smaller type with the word working, resting on the CDC and AHA bands (Pearson 2003). A result above 10 is read as acute, and ferritin from the same draw is marked as read against inflammation. CRP is a PhenoAge input and sits in the inflammation cluster of the pre-visit report. Re-test when a symptom cluster shows or six months after an intervention, never within two weeks of being unwell.
How far to trust the number
Laboratory: high-sensitivity assays are precise down to a fraction of a milligram per liter. The variation is biological: minor infections you never noticed move it, so the trend across several draws is the figure, and the first value after an illness is not a baseline.
The ranges
The laboratory’s range and flag are shown with your result and never overridden; the working optimal stays beneath, in smaller type, until the clinical advisor has reviewed it.
Sources
- Emerging Risk Factors Collaboration (Kaptoge S, et al.). C-reactive protein concentration and risk of coronary heart disease, stroke, and mortality: an individual participant meta-analysis. Lancet 2010;375:132–40. doi:10.1016/S0140-6736(09)61717-761717-7>)
- Pearson TA, et al. Markers of inflammation and cardiovascular disease: application to clinical and public health practice. A statement for healthcare professionals from the Centers for Disease Control and Prevention and the American Heart Association. Circulation 2003;107:499–511. doi:10.1161/01.cir.0000052939.59093.45
- Vitality Tracker, Recommended baseline lab panel v0.1; Lab results data model (censored values).