Blood tests: what to ask for and how to read them
The page that turns the previous three chapters into a decision: which markers answer something, when to test, how to read a value that is "within range", and what to do with each result.
Everything above rests on the same sentence: supplementing without a deficiency improves nothing. That makes the blood test the only tool that separates a useful purchase from an expensive one. This page is the practical part: what to ask for, when, and what each number means when it comes back.
This does not replace your doctor
A blood test is interpreted with the person in front of you: their history, diet, symptoms and the rest of their values. What follows is for arriving at the appointment knowing what to ask, and for not buying blind — not for diagnosing yourself or writing your own protocol.
Where the deficiencies actually are
- Unsupplemented adult vegans: low B12~52% %
Pawlak (2013). In children raised vegan without a supplement, practically all of them.
- Athletes: inadequate vitamin D56% %
Farrokhyar meta-analysis (2015), 23 studies. Higher indoors, in winter and at high latitude.
- Female runners: iron deficiency15-35% %
Range described in Sim (2019) and Clénin (2015) for female endurance athletes.
- Male athletes: iron deficiency5-11% %
Much lower, but not zero: it does show up in distance runners.
What to ask for in a first blood test
| Marker | What it answers | When it makes sense |
|---|---|---|
| Full blood count | Anaemia, cell volume, white cell series | Always. It is the baseline and usually comes as standard |
| Ferritin + CRP | Iron stores, with the inflammation corrector alongside | Always in women who train; in men, if there is unexplained fatigue |
| Transferrin and saturation | Whether available iron is reaching tissue | If ferritin is in the grey zone or there is anaemia |
| 25-OH vitamin D | Real vitamin D status | Once a year, at the end of winter. It is the one that finds the most deficiencies |
| B12 + homocysteine (or methylmalonic acid) | Functional B12 status, not just the serum level | Vegan or vegetarian diet, over 60, metformin, chronic PPI use |
| TSH (± free T4) | Thyroid function | Persistent fatigue, cold intolerance, unexplained weight change |
| Lipid panel and glucose/HbA1c | Cardiometabolic risk | General health, not performance. Make the needle count |
| Serum magnesium and zinc | Little: they barely reflect stores | Rarely. Better assessed by looking at the diet than by ordering the test |
Why ferritin is never read alone
Ferritin is an acute-phase reactant: it rises with any inflammation —a recent infection, an injury, even a very hard training block. A ferritin of 45 with high CRP can be hiding low iron stores. Without CRP alongside, the number cannot be interpreted; with it, it can.
When to do it
- 1
Outside the hardest block, with no intense session 24-48 h before
Recent exercise alters the blood count, muscle damage markers, ferritin and hepcidin. Testing the day after a long run does not measure your status: it measures the run.
- 2
Fasted and well hydrated
Dehydration concentrates blood and artificially raises haemoglobin and haematocrit. It is one of the commonest causes of odd values with no illness behind them.
- 3
Off biotin for several days beforehand
High biotin doses —very common in hair-and-nails products and in some multivitamins— skew thyroid and troponin assays. Stop it at least 48-72 h before.
- 4
At the end of winter, if what you are after is vitamin D
That is the year’s low point. A September test can read sufficient and say nothing about how you will be in March.
- 5
Repeat 8-12 weeks after starting to correct
Any earlier and stores have not had time to move. After that, maintenance once a year if everything is in place.
The five mistakes when reading a blood test
- Confusing "within range" with "optimal". The reference range is the interval where 95% of a reference population falls, not a recommendation. A ferritin of 18 prints in black and means low stores.
- Confusing an out-of-range value with a diagnosis. If you order twenty markers, the chance that one falls outside by pure chance is high. An isolated, slightly deviated value with no symptoms almost always repeats normal.
- Reading ferritin without CRP. The most expensive mistake on this list, because it leads to leaving real deficiencies untreated.
- Trusting serum magnesium or zinc. The body holds the blood level up at the expense of stores: the test can read normal with stores low. They cannot rule anything out.
- Supplementing what you have not measured. The expensive version of the same mistake: spending on a deficiency that does not exist while the one that does stays there.
Assessing nutrient status before supplementing is what the position stands recommend
Larson-Meyer’s review of nutrient status assessment in athletes lays out exactly this sequence —dietary history, then targeted testing, then supplementation if warranted— and warns of the same thing this page does: many popular markers have poor sensitivity and lead to wrong conclusions. The IOC consensus on supplements and the ACSM position stand arrive at the same place from the other end: do not recommend routine supplementation without a demonstrated need.
What to do with each result
| Result | What it means | What to do |
|---|---|---|
| 25-OH-D < 20 ng/ml | Deficiency | Medical correction protocol, then maintenance. Retest at three months |
| 25-OH-D 20-30 ng/ml | Insufficiency, the commonest winter finding | 1,000-2,000 IU/day of D3 with a fatty meal, retest at the end of winter |
| Ferritin < 15 with normal CRP | Iron deficiency | See a doctor: the cause has to be found, not only the stores refilled |
| Ferritin 15-35 with symptoms | Low stores: where the most performance is recovered | Iron on alternate days, in the morning, with vitamin C. Retest at 8-12 weeks |
| Normal ferritin with high CRP | Not conclusive: inflammation inflates it | Repeat during a quiet period before deciding anything |
| Low or borderline B12 with high homocysteine | Functional deficiency even if serum looks acceptable | Supplement, and work out whether the problem is intake or absorption |
| Everything normal and you are still tired | The problem is probably not a micronutrient | Look first at sleep, training load and calorie intake. No tub fixes that |
What if the problem is that you eat too little?
It is the commonest cause of several deficiencies at once, and a multivitamin does not fix it. Work out your expenditure and check what you are actually eating before buying anything.
- Your dataWeight, height, age
- ActivitySessions per week
- ResultExpenditure and target
Takeaways
- Ask for few, targeted markers: full blood count, ferritin with CRP, 25-OH vitamin D, and B12 if your diet or age calls for it.
- Test away from a hard session, fasted, and off biotin for 48-72 h.
- "Within range" is not "optimal", and an isolated out-of-range value is not a diagnosis.
- Ferritin without CRP cannot be interpreted.
- Serum magnesium and zinc cannot rule out a deficiency.
- If everything is normal and you are still tired, the next place to look is sleep and calories, not the shop.
With the result in hand
Each value has its chapter, and the buying decision has its own.
Sources for this chapter
- [1]Larson-Meyer DE, Woolf K, Burke L (2018). Assessment of Nutrient Status in Athletes and the Need for Supplementation. International Journal of Sport Nutrition and Exercise Metabolism. review
- [2]Maughan RJ, Burke LM, Dvorak J, et al. (2018). IOC consensus statement: dietary supplements and the high-performance athlete. British Journal of Sports Medicine. position stand
- [3]Thomas DT, Erdman KA, Burke LM (2016). Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise. position stand
- [4]Farrokhyar F, Tabasinejad R, Dao D, et al. (2015). Prevalence of vitamin D inadequacy in athletes: a systematic review and meta-analysis. Sports Medicine. meta-analysis
- [5]Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism. official guideline
- [6]Clénin G, Cordes M, Huber A, et al. (2015). Iron deficiency in sports — definition, influence on performance and therapy. Swiss Medical Weekly. position stand
- [7]Sim M, Garvican-Lewis LA, Cox GR, et al. (2019). Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology. review
- [8]Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D (2013). How prevalent is vitamin B12 deficiency among vegetarians?. Nutrition Reviews. review
- [9]Allen LH (2009). How common is vitamin B-12 deficiency?. American Journal of Clinical Nutrition. review