A fasting blood test means eight to twelve hours without food and water only. But if you train, fasting is not the variable that moves your results most. Yesterday session, your creatine, your supplements and how much fluid you lost shift your values further than breakfast ever would.
That is exactly why healthy athletes with perfectly good kidneys get a high creatinine back.
Below are nine things that skew your results. For each one you get the value that moves, the direction, how long it lasts and what to do about it. Seven of the nine have nothing to do with food.
| What you did | What moves | Direction | How long |
|---|---|---|---|
| Hard training | CK, AST, ALT | up | 2 to 7 days |
| Taking creatine | creatinine, eGFR | creatinine up, eGFR down | while you supplement |
| Dehydration or sauna | haematocrit, Hb, sodium | up | until you rehydrate |
| Taking biotin | TSH down, free T4 up | assay error, not a real change | 2 to 3 days |
| Race or long endurance effort | ferritin, hs-CRP | up | several days |
| Alcohol | GGT, triglycerides | up | days to weeks |
| Anti-inflammatories | creatinine, eGFR | creatinine up | around use |
| Drawing late in the day | testosterone, cortisol | down | every single day |
1. Your hard session yesterday
A hard session raises your creatine kinase, and with it often your AST and ALT. Those last two are known as liver values, but they sit in muscle tissue just as much. After a heavy leg session or a movement you have not done in months, your CK can stay elevated for days, peaking between 24 and 72 hours.
That is muscle damage, not liver damage.
The largest rise comes from eccentric loading: the braking part of a movement, such as lowering into a squat or running downhill. Repeat the same session and the response is clearly smaller. Your muscle adapts (PMID 1569847). The recovery process itself runs over days rather than hours (PMID 20010127).
What to do: draw at least 48 to 72 hours after your last hard session, and longer after a race or an unfamiliar workout. What your value means afterwards sits in high CK.
2. Creatine
Creatine raises your blood creatinine without anything being wrong with your kidneys. Creatinine is simply the breakdown product of creatine, so more supply gives a higher value. Because eGFR is calculated from creatinine, it falls along with it. On paper that reads as reduced kidney function.
Your kidney function has not changed. The equation estimating it has.
This is well studied by now. A systematic review and meta-analysis of creatine supplementation found no adverse effect on renal function (PMID 31375416), and a later review concluded that the notion of creatine-induced kidney failure is due for revision (PMID 36986197).
What to do: mention that you supplement creatine, or add cystatin C. That value does not come from your muscle mass and is not affected by creatine. More in is creatine bad for your kidneys, high creatinine from muscle mass and cystatin C.
3. Dehydration and the sauna
When you lose fluid your blood becomes more concentrated. There is less plasma around the same number of cells and proteins, so everything measured per litre reads higher. Your haematocrit, haemoglobin, total protein and sodium all shift up without anything having been added.
Your blood did not get thicker. It just holds less water.
The effect is measurable after ordinary exercise: sequential cycling and running produced clear haemoconcentration despite fluid intake (PMID 3574046). A sauna, a hot race day or an evening of heavy sweating does the same.
What to do: drink normally the day before and the morning itself, and schedule no sauna or hard session in the 24 hours before your draw. Water does not break your fast. When a high haematocrit does mean something sits in high haematocrit.
4. Biotin in your supplements
Biotin, also known as vitamin B7, interferes with the measurement itself. Many laboratory hormone tests rely on a biotin-streptavidin binding step, and a high dose of biotin in your blood interferes with it. The result is a value that looks abnormal while your hormones are normal.
That is not a real change. That is an assay error.
The classic pattern: your TSH reads falsely low and your free T4 falsely high, which together look like an overactive thyroid. Research has demonstrated that interference in thyroid function tests directly (PMID 32118725), and a review of interferences in thyroid immunoassays describes the same mechanism and how to detect it (PMID 29982406).
Where it hides: hair, skin and nail supplements, high-dose B complexes and many pre-workouts. The doses in those sit far above anything you get from food.
What to do: stop biotin 48 to 72 hours before your draw, and report it if you did take it. If you get a thyroid result that does not match your symptoms, this is the first thing to rule out.
5. A race or long endurance effort
Ferritin is not only your iron store, it is also an acute phase protein. It rises with inflammation or heavy load. Draw shortly after a race and a ferritin that is genuinely too low can look normal. An iron deficiency disappears from view at exactly the moment you are looking for it.
This is the skew with the most annoying consequence, because you miss something treatable.
Heavy exercise demonstrably affects iron metabolism during recovery (PMID 20175428), and in ultramarathon runners iron and anaemia-related markers differ by race distance (PMID 36800687). Your hs-CRP moves in the same direction.
What to do: schedule your draw in a quiet week, not in the days after a race or a long endurance block. Run hs-CRP alongside it so you can see whether inflammation pulled your ferritin up. Why endurance athletes in particular are at risk sits in iron deficiency in athletes.
6. Last night alcohol
Alcohol raises your gamma-GT and your triglycerides. GGT is among the most sensitive values for alcohol intake and responds to a period of drinking more than usual, not only to years of heavy use (PMID 11563810). Other liver values can move with it.
A wedding at the weekend is enough to colour your liver panel.
In the context of alcohol markers GGT is used as an indicator for exactly that reason, with the caveat that the value also rises from other causes and therefore never stands alone (PMID 17045579).
What to do: avoid alcohol for at least 72 hours before your draw, and longer if you are having a liver panel done. What drinking does to your recovery and your liver sits in alcohol and your blood values.
7. Anti-inflammatories
Ibuprofen, naproxen and diclofenac affect blood flow through your kidneys. At rest that effect is small, but around hard exercise, heat and fluid loss it adds up. Your creatinine can then come out higher than your baseline.
The problem is not the pill itself, but the combination with a hard effort.
The effect of anti-inflammatories on renal function has been studied directly in athletes (PMID 15870622), and during a mountain ultramarathon their use was mapped alongside renal function and hyponatraemia (PMID 17304005).
What to do: skip anti-inflammatories in the days before your draw where that is reasonable, and otherwise report them. Never stop prescribed medication on your own: discuss that with your doctor. A complete picture of your kidneys comes from kidney health.
8. The time of your draw
Testosterone and cortisol follow a daily rhythm. Testosterone is highest in the morning and falls through the day, so the same man can have a normal value at eight and a low one at four. An afternoon draw gives you a different answer than a morning draw.
With hormones, the time is part of the measurement.
That diurnal variation has been studied explicitly in men and proved clinically relevant for interpreting testosterone and other sex hormones (PMID 19088162). Cortisol follows a comparable pattern, peaking shortly after waking.
What to do: draw hormones before ten in the morning, and repeat any follow-up at roughly the same time so your values stay comparable. More in how to test testosterone and cortisol and stress.
9. What fasting does and does not break
Water does not break your fast and you should drink it. Brushing your teeth does not break it either, because you do not swallow the toothpaste. Black coffee is the awkward one: it carries almost no calories, but it does affect your metabolism and your blood sugar, so leave it until after the draw.
Water yes, toothpaste yes, coffee better not.
For blood lipids, fasting is often not necessary at all. Non-fasting lipids predict cardiovascular risk just as well, and internationally non-fasting measurement has become the default for most situations (PMID 30522787). For glucose and insulin, fasting remains the standard.
What to do: follow the instruction that came with your test rather than a general rule. Which values run together in a broad panel is visible at 360 health.
How to plan your draw
Turn the nine points into a calendar and it becomes simple. Pick a quiet week with no race. Put your last hard session at least 48 hours before. Skip alcohol and biotin for the last three days. Drink water normally. Draw before ten in the morning.
Do one thing this week: check the label of your hair, skin and nail supplement or your pre-workout for biotin. That is the skew that most often goes unnoticed.
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
References
- Clarkson PM, et al. Muscle function after exercise-induced muscle damage and rapid adaptation. Medicine and Science in Sports and Exercise, 1992. PMID 1569847.
- Paulsen G, et al. Time course of leukocyte accumulation in human muscle after eccentric exercise. Medicine and Science in Sports and Exercise, 2010. PMID 20010127.
- de Souza e Silva A, et al. Effects of creatine supplementation on renal function: a systematic review and meta-analysis. Journal of Renal Nutrition, 2019. PMID 31375416.
- Longobardi I, et al. Is it time for a requiem for creatine supplementation-induced kidney failure? A narrative review. Nutrients, 2023. PMID 36986197.
- Wells CL, et al. Fluid shifts with successive running and bicycling performance. Medicine and Science in Sports and Exercise, 1987. PMID 3574046.
- Zhang Y, et al. Assessment of biotin interference in thyroid function tests. Medicine (Baltimore), 2020. PMID 32118725.
- Favresse J, et al. Interferences with thyroid function immunoassays: clinical implications and detection algorithm. Endocrine Reviews, 2018. PMID 29982406.
- Peeling P, et al. Effects of exercise on hepcidin response and iron metabolism during recovery. International Journal of Sport Nutrition and Exercise Metabolism, 2009. PMID 20175428.
- Shin KA. Comparison of the differences in iron and anemia-related markers by ultramarathon distance. Journal of Sports Medicine and Physical Fitness, 2023. PMID 36800687.
- Whitfield JB. Gamma glutamyl transferase. Critical Reviews in Clinical Laboratory Sciences, 2001. PMID 11563810.
- Niemela O. Biomarkers in alcoholism. Clinica Chimica Acta, 2007. PMID 17045579.
- Baker J, et al. Effects of indomethacin and celecoxib on renal function in athletes. Medicine and Science in Sports and Exercise, 2005. PMID 15870622.
- Page AJ, et al. Exercise-associated hyponatremia, renal function, and nonsteroidal antiinflammatory drug use in an ultraendurance mountain run. Clinical Journal of Sport Medicine, 2007. PMID 17304005.
- Brambilla DJ, et al. The effect of diurnal variation on clinical measurement of serum testosterone and other sex hormone levels in men. Journal of Clinical Endocrinology and Metabolism, 2009. PMID 19088162.
- Langsted A, Nordestgaard BG. Nonfasting versus fasting lipid profile for cardiovascular risk prediction. Pathology, 2019. PMID 30522787.
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