What Is the Difference Between CRP and hs-CRP?
CRP and hs-CRP measure the same protein. hs-CRP reads lower, often near 0.3 mg/L, so it catches low-grade inflammation that many standard CRP assays report only as below their limit.
CRP and hs-CRP measure the same C-reactive protein. The difference is the assay: hs-CRP is a high-sensitivity version that can quantify low-grade inflammation at lower concentrations than many standard CRP assays. Many standard assays have limited sensitivity at low concentrations and the reportable lower limit varies by lab, so a standard result below that limit may be returned as something like <5 mg/L rather than a number.
What is the difference between hs-CRP and CRP?
The hs stands for high-sensitivity, not a different marker. Both tests measure C-reactive protein, a liver protein released when macrophages and other immune cells signal through interleukin-6. Standard CRP and hs-CRP use the same target but have different lower limits of quantification.
The difference is not the protein. It is how low the test can read.
A standard CRP is often used to track acute inflammation, such as pneumonia, a lupus flare, or a post-surgical infection. It is reported in mg/L or mg/dL, and the lower reportable limit varies by assay and lab. Below that limit, the result may be returned as something like <5 mg/L or <0.5 mg/dL, depending on the assay, rather than a number.
hs-CRP has a lower reportable limit often near 0.3 mg/L as an example; some labs and assays report lower. That extra sensitivity matters because the AHA/CDC hs-CRP bands for cardiovascular risk are below 1 mg/L, 1 to 3 mg/L, and above 3 mg/L. Low-grade inflammation can also be below 1 mg/L, so a standard assay that reports only a floor may not place you within those bands, while an hs-CRP assay can return a number such as 3.2. Neither result is wrong; they are resolving different parts of the same scale.
What you see on a lab report
You might see any of these: - Standard CRP: <5 mg/L, <0.5 mg/dL, or a number like 28 mg/L during illness, depending on the assay and lab. - hs-CRP: 1.4 mg/L, <0.3 mg/L, or 0.6 mg/L.
The units conversion is straightforward: 1 mg/dL equals 10 mg/L. A standard CRP of 0.8 mg/dL is 8 mg/L.
| Assay | Typical lower limit | Used for |
|---|---|---|
| Standard CRP | Varies by lab and assay; may return <5 mg/L or <0.5 mg/dL as an example | Acute infection and inflammatory disease |
| hs-CRP | Often near 0.3 mg/L; varies by lab and assay | Low-grade inflammation and cardiovascular risk bands |
Scroll the table sideways for the rest →
Lower limits vary by lab and assay. Any numeric examples are illustrative, not universal limits.
The most common mistake is treating a standard CRP below its reportable limit as if it were zero. It is not. It means below the reportable limit of a less sensitive assay. If you are looking at low-grade inflammation, that standard result cannot tell you whether you are at 0.4 or 4.8. An hs-CRP result can.
Why the lower limit matters for longevity
Low-grade chronic inflammation is one of the most consistent findings in aging research. C-reactive protein is not a bystander; it is a downstream readout of ongoing IL-6 signalling from stressed or damaged tissue. The American Heart Association and CDC settled on hs-CRP bands for cardiovascular risk in otherwise healthy people: below 1 mg/L is low, 1 to 3 mg/L is average, above 3 mg/L is high (Pearson et al. 2003 Circulation, AHA/CDC). Those bands only work with a high-sensitivity assay.
The same marker feeds the PhenoAge biological age estimate, the Levine 2018 Aging model that combines age with nine routine blood markers, including C-reactive protein (Levine et al. 2018 Aging). HealthOS treats CRP or hs-CRP as one of the nine PhenoAge markers. The free PhenoAge calculator accepts either marker, and it can fill from your /blood report if you already have one. If your report gives only a floor below the lab's reportable limit, that value is less informative for PhenoAge because it remains a floor rather than an exact number, but CRP and hs-CRP can both work when the report includes a numeric result.
What raises it that has nothing to do with disease
CRP is fast and non-specific. It can rise within hours after an insult, peak around a day or two later, and fall over several days once the trigger clears. That means a single high hs-CRP does not always mean chronic inflammation. It can reflect: - a cold or sinus infection you were not aware of - dental infection or gum disease - intense exercise in the day or two before the draw - surgery, injury or vaccination - poor sleep or heavy alcohol intake
For a baseline or PhenoAge number, the practical rule is to test when you are well. If you had an infection, surgery, injury, vaccination or a very hard workout, discuss when to repeat the test with your doctor, because CRP can stay elevated for days to weeks after the trigger clears. A value above 10 mg/L often points to an acute process rather than a stable low-grade state, so reading it as cardiovascular risk is misleading.
Which test should you get?
If you want to track low-grade inflammation or complete a PhenoAge marker set, hs-CRP is the more useful test. If a doctor is looking at an infection, an autoimmune flare, or whether treatment for an inflammatory condition is working, standard CRP is usually enough.
The usual follow-up question is whether a standard CRP can be used instead. You can, if it gives an actual number above its lower limit. If it reports below the lab's reportable limit, you cannot place yourself on the AHA/CDC hs-CRP bands below 1 mg/L, 1 to 3 mg/L, or above 3 mg/L. You need hs-CRP for that.
If you already have a recent report, upload it at healthosx.com/blood or use the free PhenoAge calculator first. If you do not have a recent report or your current panel is missing hs-CRP, you can order a lab test at healthosx.com/blood/test.
How to read your own result
Start by checking the units and the assay. Then: - hs-CRP below 1 mg/L: low cardiovascular risk band, when you are otherwise well. - hs-CRP 1 to 3 mg/L: average band; worth watching alongside other markers. - hs-CRP above 3 mg/L: high band, but rule out a recent infection first. - Any CRP above 10 mg/L: more likely acute inflammation than a stable aging signal. Worth discussing with a doctor if you cannot explain it.
These are risk bands, not diagnoses. A low result does not promise anything, and a high result from a week with a cold is expected.
Upload a lab report at healthosx.com/blood and the report reader explains CRP or hs-CRP against ranges matched to your age and sex. If your report has the nine PhenoAge markers, the same screen works out your phenotypic age, and a second upload later shows how each marker moved.
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HealthOS Research