hs-CRP test: fasting, timing and what affects it

Updated October 31, 2026 · 13 sources · Written by the AumniVIDA team

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Short answer

The hs-CRP test is an ordinary blood draw and does not need fasting. Because single values vary by about 40%, guidelines advise testing while well, averaging two results about 2 weeks apart, and repeating any result over 10 mg/L (1.0 mg/dL). Avoid testing within a few days of illness or a hard race.

How an infection spikes CRP and how long it takes to settle

Rise
Hour 0Trigger: infection, injury or surgeryThe liver starts making extra CRP.
About 6 hoursCRP passes 5 mg/L
About 48 hoursCRP peaksSevere infection or trauma can push it past 500 mg/L.
Settle
Days afterCRP falls once the trigger endsIts half-life is about 19 hours, so the level drops within days.
About 2 weeksRetest windowIf a result was over 10 mg/L, repeat the test now (CDC/AHA 2003).

A marathon gives a smaller rise: CRP peaked the next day and was still raised 3 days later (Takayama 2018). Timings are typical averages (Pepys 2003).

CRP passes 5 mg/L about 6 hours after a trigger, peaks around 48 hours and, with a half-life near 19 hours, falls back once the trigger ends; that is why a result over 10 mg/L is repeated after about 2 weeks.

Key takeaways

1
Fasting or nonfasting results are equivalent, and CRP shows little daily or seasonal rhythm [1] [2].
2
Within-person variation is about 42%, so a change needs to be over 100% to be clearly real [3].
3
Average two tests about 2 weeks apart, taken while metabolically stable [1].
4
A marathon raises CRP for about 3 days; infection raises it until the trigger resolves [4] [5].

What is the hs-CRP test?

It is a blood test that measures C-reactive protein precisely at low levels, from about 0.1 to 10 mg/L. It uses a normal sample from a vein, and results are reported in mg/L.

Labs use antibody-based methods, such as immunoturbidimetry or immunonephelometry, that can read CRP from about 0.01 to 10 mg/L [2]. The CDC/AHA statement asked that results be expressed in mg/L only [1].

The test is often ordered with a lipid panel. It adds the most for people at intermediate risk, and the guidance was not to screen the whole adult population [1]. The 2025 ACC statement argues for broader use alongside LDL cholesterol [6] [7].

Do you need to fast for an hs-CRP test?

No. Fasting and nonfasting results are equivalent, and CRP does not change with meals. If the same draw includes a fasting test, such as fasting glucose or insulin, follow the instructions for that test.

The CDC/AHA statement specifies that hs-CRP can be measured fasting or nonfasting [1]. CRP shows no diurnal variation and no variation with food intake [2], and there is little seasonal variation [1]. Time of day therefore matters less than your health in the days before the draw.

Does hs-CRP fluctuate?

Yes, more than most blood tests. In healthy people a single value varied by about 42% from one test to the next, and two results had to differ by 118% before the change was likely real.

A study of healthy subjects measured the sources of variation [3]:

Analytical variation from the lab method: about 5%.
Within-person variation: about 42%.
Between-person variation: about 93%.

Because people differ from each other much more than they differ from week to week, CRP can still rank people reliably over time [3]. This is why the guidance is to average two tests and why two measurements are enough to classify risk, much as with cholesterol [1].

The CDC/AHA statement also advised against serial hs-CRP testing to monitor the effect of treatment [1]. If you retest to follow a lifestyle change, compare averages over months, not single results.

When should you not take an hs-CRP test?

Do not test during or soon after an infection, injury, surgery or inflammatory flare, or within about 3 days of a very hard endurance effort. If a result is over 10 mg/L, wait about 2 weeks and test again.

Guidelines say people with an active infection, systemic inflammation or trauma should not be tested until these have settled [1]. After a trigger, CRP passes 5 mg/L within about 6 hours and peaks near 48 hours [5]. After a marathon, it peaked the next day and was still raised 3 days later [4].

Things worth noting before the draw:

Any cold, flu, dental infection or injury in the past 2 weeks.
A race or unusually hard training session in the past 3 days.
Starting or stopping estrogen or progestogen therapy [1].
A flare of a chronic inflammatory condition [1].

How an infection spikes CRP and how long it takes to settle

Rise
Hour 0Trigger: infection, injury or surgeryThe liver starts making extra CRP.
About 6 hoursCRP passes 5 mg/L
About 48 hoursCRP peaksSevere infection or trauma can push it past 500 mg/L.
Settle
Days afterCRP falls once the trigger endsIts half-life is about 19 hours, so the level drops within days.
About 2 weeksRetest windowIf a result was over 10 mg/L, repeat the test now (CDC/AHA 2003).

A marathon gives a smaller rise: CRP peaked the next day and was still raised 3 days later (Takayama 2018). Timings are typical averages (Pepys 2003).

Who should get an hs-CRP test?

Guidelines differ. The 2003 CDC/AHA statement favored testing people at intermediate heart risk and not screening everyone, the ESC says CRP adds little, and the 2025 ACC statement calls for broad testing alongside LDL cholesterol.

CDC/AHA, 2003: optional for people at 10% to 20% ten-year coronary risk; not for screening the whole population [1].
AHA/ACC, 2018: 2.0 mg/L or more is a risk-enhancing factor when measured [8].
ESC, 2021: CRP has limited additional value; routine biomarker testing not advised [9].
ACC, 2025: broad screening in primary and secondary prevention, together with LDL cholesterol [6] [7].

People who already have heart disease are a special case: in statin-treated patients hs-CRP predicted future events more strongly than LDL cholesterol [10]. Whether testing changes your care is a question for your clinician.

What can make an hs-CRP result misleading?

Recent infection or injury, a hard race, hormone therapy, statins and a mix-up between mg/L and mg/dL are the usual culprits. A standard CRP result reported as "under 3" or "under 5" also cannot be read on the heart-risk scale.

Issue	Effect on the result	Source
Infection, injury, surgery in the past 2 weeks	Falsely high for heart risk, often over 10 mg/L	[5] [1]
Marathon or very hard session in the past 3 days	Raised	[4]
Estrogen or progestogen therapy	Raised	[1]
Statin treatment	Lowered by about 17% to 37%	[11] [12]
Result printed in mg/dL	Looks 10 times smaller than the mg/L categories	[1]
Standard instead of hs assay	Cannot read below about 3 mg/L	[13]

If one of these applies, note it next to the result, or repeat the test once it no longer does.

More about hs-CRP

Protocols that act on hs-CRP

Common questions

What is the difference between a CRP test and an hs-CRP test?

Both measure the same protein, but the hs test reads the low range, down to about 0.1 mg/L, needed for heart risk [13]. See hs-CRP vs CRP.

Can I drink coffee before an hs-CRP test?

CRP does not change with meals [2], so a normal breakfast or coffee does not affect it. Follow the instructions for any other test in the same draw.

How long does it take for CRP to go down after an illness?

With a half-life of about 19 hours [5], CRP falls within days once the illness resolves. Guidelines suggest retesting about 2 weeks after a high result [1].

Should I exercise before an hs-CRP test?

Avoid a very hard or long session in the 3 days before. After a marathon, CRP stayed raised for 3 days [4].

How often should hs-CRP be tested?

No guideline sets a schedule. Two tests about 2 weeks apart give a baseline [1]; beyond that, your clinician decides whether repeat testing would change anything.

Sources

1.Pearson TA, Mensah GA, Alexander RW, 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. doi:10.1161/01.cir.0000052939.59093.45
2.Kamath DY, Xavier D, Sigamani A, Pais P. High sensitivity C-reactive protein (hsCRP) & cardiovascular disease: An Indian perspective. Indian Journal of Medical Research, 2015. doi:10.4103/0971-5916.166582
3.Macy EM, Hayes TE, Tracy RP. Variability in the measurement of C-reactive protein in healthy subjects: implications for reference intervals and epidemiological applications. Clinical Chemistry, 1997. PubMed 8990222
4.Takayama F, Aoyagi A, Takahashi K, Nabekura Y. Relationship between oxygen cost and C-reactive protein response to marathon running in college recreational runners. Open Access Journal of Sports Medicine, 2018. doi:10.2147/OAJSM.S183274
5.Pepys MB, Hirschfield GM. C-reactive protein: a critical update. Journal of Clinical Investigation, 2003. doi:10.1172/JCI18921
6.Mensah GA, Arnold N, Prabhu SD, Ridker PM, Welty FK. Inflammation and Cardiovascular Disease: 2025 ACC Scientific Statement: A Report of the American College of Cardiology. Journal of the American College of Cardiology, 2025. doi:10.1016/j.jacc.2025.08.047
7.TCTMD. ACC: It's Time for Clinicians to Address Inflammation in CVD. TCTMD (Cardiovascular Research Foundation), 2025. Link
8.Grundy SM, Stone NJ, Bailey AL, et al. 2018 AHA/ACC/AACVPR/AAPA/ABC/ACPM/ADA/AGS/APhA/ASPC/NLA/PCNA Guideline on the Management of Blood Cholesterol: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines. Circulation, 2019. doi:10.1161/CIR.0000000000000625
9.Visseren FLJ, Mach F, Smulders YM, et al. 2021 ESC Guidelines on cardiovascular disease prevention in clinical practice. European Heart Journal, 2021. doi:10.1093/eurheartj/ehab484
10.Ridker PM, Bhatt DL, Pradhan AD, et al. Inflammation and cholesterol as predictors of cardiovascular events among patients receiving statin therapy: a collaborative analysis of three randomised trials. Lancet, 2023. doi:10.1016/S0140-6736(23)00215-5
11.Albert MA, Danielson E, Rifai N, Ridker PM. Effect of statin therapy on C-reactive protein levels: the pravastatin inflammation/CRP evaluation (PRINCE): a randomized trial and cohort study. JAMA, 2001. doi:10.1001/jama.286.1.64
12.Ridker PM, Danielson E, Fonseca FA, et al. Rosuvastatin to prevent vascular events in men and women with elevated C-reactive protein. New England Journal of Medicine, 2008. doi:10.1056/NEJMoa0807646
13.Grootendorst DC, de Jager DJ, Brandenburg VM, et al. Excellent agreement between C-reactive protein measurement methods in end-stage renal disease patients--no additional power for mortality prediction with high-sensitivity CRP. Nephrology Dialysis Transplantation, 2007. doi:10.1093/ndt/gfm381