It depends entirely on the marker, and the range is enormous. Triglycerides respond within weeks. HbA1c reflects roughly three months and cannot usefully be retested sooner. DEXA is annual at most. Lp(a) is once in your life. Retesting faster than the underlying biology moves does not produce a trend, it produces noise you will misread as change.
Match the retest interval to the biology
Every marker has a timescale built into it, set by the thing it measures. HbA1c averages three months of glucose because that is roughly how long a red blood cell lives. Triglycerides turn over in days. Bone density changes over years.
Retest faster than that timescale and you are not measuring change, you are measuring the assay's variability plus whatever you ate on Tuesday. The interval is not an administrative convention, it is a property of the marker.
If bloodwork is not accessible, tracking without bloodwork covers what a tape measure and a floor can tell you.
Intervals by marker
| Marker | While actively changing it | Routine monitoring | Why that interval |
|---|---|---|---|
| Triglycerides | 4–8 weeks | 6–12 months | Fastest-moving lipid; responds to alcohol and sugar within weeks |
| Resting heart rate | Continuous, read monthly | Continuous, read monthly | Daily noise is large; the monthly average is the signal |
| HbA1c | 3–6 months | 6–12 months | Averages roughly three months of glycaemia by construction |
| ApoB | 3–6 months | 6–12 months | Reflects a slow hepatic equilibrium, not last week’s meals |
| hs-CRP | 3–6 months, when well | 6–12 months | Any infection or hard session distorts it; retest away from both |
| Functional tests | 3–6 months | 6–12 months | Real improvement is slower than measurement noise |
| VO2 Max | 3–6 months | 6–12 months | Meaningful adaptation takes a training block, not a fortnight |
| Body fat, tape method | Fortnightly at most | Quarterly | 3.5-point standard error swamps a week of genuine change |
| DEXA | 6–12 months | Annually at most | Expected change falls inside the least significant change below that |
| Lp(a) | Once, ever | Once, ever | Largely genetic and stable through adult life |
These are Longevity Coach IQ guidance intervals, drawn from the measurement timescales cited below rather than from a single published schedule.
What happens when you retest too often
Two failures, and they pull in opposite directions.
Each marker carries two intervals. The first applies while you are actively working on that marker and want to know whether the change is landing. The second applies once the number has been stable for a while and you are monitoring rather than intervening. Someone eight weeks into a serious dietary change has a reason to retest HbA1c at three months; someone whose numbers have held for two years does not.
You read noise as progress. A body fat estimate with a 3.5 percentage point standard error will happily report a two-point "improvement" between Monday and Friday. Nothing changed. You will nonetheless attribute it to whatever you started that week, and carry a false belief forward.
You abandon things that were working. A four-week HbA1c retest after a genuine dietary change returns a number still dominated by the previous eight weeks. It reads as failure. People stop, weeks before the change would have shown.
Both errors come from the same root, and both are avoidable by waiting.
How to keep two measurements comparable
An interval only means something if the two measurements are comparable. Same time of day, same fasting state, same method.
For blood markers that means a genuine 9 to 12 hour fast and, for triglycerides specifically, at least 48 hours without alcohol rather than merely overnight. For body composition it means the same tape and the same technique, since switching from a tape to a smart scale produces a change in the number that has nothing to do with your body.
Record the method alongside the result. A year later you will want to know whether you are comparing like with like, which is what the tracking history inside the longevity app is for.
When to break a retest interval
Retest sooner if a result was implausible and you suspect an error, if you have started a medication that acts directly on the marker, or if a clinician asks you to. Those are real reasons.
Curiosity is not one, and neither is impatience two weeks into a change. If the urge to retest is strong, the more useful move is to go and do the thing that would move the marker.
Sources
Key references for the claims on this page. Where a figure is attributed to a specific study or body, it is named here.
- American Diabetes Association Standards of Care: HbA1c reflects average glycaemia over approximately three months, corresponding to erythrocyte lifespan, and is recommended at intervals of three to six months depending on stability.
- Koschinsky ML, Bajaj A, Boffa MB, et al. A focused update to the 2019 NLA scientific statement on use of lipoprotein(a) in clinical practice. Journal of Clinical Lipidology, 2024. The once-in-a-lifetime measurement recommendation.
- International Society for Clinical Densitometry. Official Positions on Adult Densitometry. Least significant change is a defined term in that guidance: the smallest difference between two scans that can be distinguished from measurement precision error.
Frequently asked
Why can I not retest HbA1c monthly?
Because it is a weighted average over the roughly three-month lifespan of a red blood cell. A test four weeks after a change is still dominated by the eight weeks preceding it, so the number you get mostly reflects the period you were trying to move on from.
If bloodwork is not accessible, tracking without bloodwork covers what a tape measure and a floor can tell you.
My triglycerides dropped 40 points in three weeks. Is that real?
Quite possibly, yes. Triglycerides are the fastest-moving lipid, and cutting alcohol and added sugar produces genuine change on that timescale. It is one of the few markers where a short interval is informative rather than misleading.
Should I retest Lp(a) after changing my diet?
No. It is largely genetic and stable across adult life, and a second test will return approximately your first result while charging you again. Once is the correct number of times.
How often should I redo the functional tests?
Every three to six months. Sit-to-stand, grip and the walk test all improve with training but not week to week, and frequent maximal testing mostly adds fatigue. The Trail Making Test additionally suffers a practice effect, so a first attempt is often the most informative one.