How to prepare for the «Hemoglobin» test: rules for taking it
Hemoglobin is the protein of red blood cells that carries oxygen from the lungs to the tissues, and the main laboratory criterion for anemia. It is easy to have tested: the indicator is part of any complete blood count. It is harder to obtain a result on which a correct decision can be made — to begin an evaluation for iron deficiency, to change a training plan or, conversely, to relax. Our editorial team explains what affects hemoglobin the day before and on the day of the test and how to avoid common mistakes.
Why hemoglobin is tested and what it shows
Each red blood cell contains hundreds of millions of hemoglobin molecules, and each of them has at its center an iron atom capable of binding oxygen. The concentration of hemoglobin in the blood, which the laboratory reports in grams per liter, reflects the oxygen capacity of the blood — how much oxygen the blood can deliver to the muscles, brain and heart.
The most common cause of decreased hemoglobin worldwide is iron deficiency. Women of childbearing age, pregnant women, vegetarians and also endurance athletes are especially susceptible to it, as iron is lost through sweat, through microbleeds in the digestive tract and through the destruction of red blood cells during impact loads.
For an athlete, even a moderate decrease in hemoglobin means a drop in endurance, faster fatigue and slower recovery. That is why it is useful to have the test done not only when there are complaints but also on a scheduled basis — at the start of the season and during periods of high training volume.
It is important to understand that hemoglobin is an indicator of «consequence». Iron reserves are first depleted in the depot (this is reflected by ferritin), and only then does hemoglobin fall. Therefore, when iron deficiency is suspected, the doctor usually orders hemoglobin together with ferritin and other iron metabolism indicators.
Where to draw blood: vein or finger
Hemoglobin can be determined in venous blood at the laboratory or in capillary blood — from a finger, including with portable analyzers. For diagnosis and monitoring over time, venous blood is preferred: it gives more reproducible results.
In a capillary sample the result is affected by technique: if the finger is squeezed hard, tissue fluid mixes into the blood and hemoglobin comes out underestimated. Cold hands, the first drop of blood, an insufficient puncture — all of this increases the error. The WHO in its guidance on hemoglobin thresholds draws attention to the importance of the sample source and the quality of measurement.
| Parameter | Venous blood | Capillary blood |
|---|---|---|
| Reproducibility | High | Lower, depends on technique |
| Typical errors | Prolonged tourniquet, wrong tube | Squeezing the finger, cold hands |
| Use | Diagnosis, treatment monitoring | Screening, blood donation, field conditions |
| Additional indicators | Complete blood count, red cell indices | Usually hemoglobin only |
If you are monitoring hemoglobin over time, choose one method of collection and one laboratory. It is incorrect to compare the result of a portable hemoglobinometer with a laboratory test of venous blood.
During a venous blood draw, standard requirements apply: a few minutes of quiet sitting, a tourniquet for no longer than about a minute, without active «fist work». These details are described in more detail in our article on hematocrit, because both indicators respond to them similarly.
What to do the day before
No special diet is needed before a hemoglobin test: one plate of meat or buckwheat will not change it overnight. However, the complete blood count is customarily taken in the morning on an empty stomach or after a light breakfast, so that the other indicators on the same form are also correct.
- give blood in the morning, at the same time on repeat tests;
- the day before, drink water as usual, without excessive «loading» with fluid;
- do not consume alcohol for 24 hours;
- avoid exhausting workouts for 24–48 hours and do not train before the test;
- do not smoke for at least an hour before the blood draw.
The concentration of hemoglobin, like hematocrit, depends on plasma volume. Severe dehydration artificially raises it, while drinking large amounts of fluid or intravenous infusions lower it. The usual drinking regimen is the best option.
Intense, prolonged exertion, especially running on a hard surface, can cause the destruction of part of the red blood cells. The day before the test it is better to limit yourself to a recovery workout in order to assess the «baseline» level.
Special situations: menstruation, blood donation, medications
During heavy menstruation, hemoglobin can temporarily decrease, and in women with long, heavy periods chronic blood loss is one of the main causes of iron-deficiency anemia. For scheduled monitoring it is more convenient to have the test done outside of menstruation, but if the doctor is looking for the cause of anemia, the nature of the cycle should be described in detail.
After a blood donation, hemoglobin is restored over a few weeks, and iron reserves take even longer. If you have recently been a donor, inform the doctor: a low result in that case is expected.
Iron preparations do not affect hemoglobin itself instantly, since new red blood cells are formed gradually. However, if serum iron is ordered together with hemoglobin, taking a tablet the evening or morning before will raise it substantially. It is usually advised to take a break from iron before a test for iron metabolism indicators — the length of the pause is determined by the doctor.
Androgens, anabolic steroids, erythropoietin and other stimulators of erythropoiesis raise hemoglobin. Their non-medical use is banned in sport by the World Anti-Doping Agency and is associated with the risk of blood thickening and thrombosis. The doctor must know about the intake of any hormonal drugs, otherwise the interpretation will be incorrect.
Interpretation and anemia thresholds
In a 2011 document, the WHO defined the following anemia thresholds for adults at sea level: hemoglobin below 130 g/L in men, below 120 g/L in non-pregnant women and below 110 g/L in pregnant women. Updated WHO guidelines from 2024 revised some of the thresholds and the methods of adjustment, so the specific limits on the form depend on the laboratory and the standards adopted in the country.
The WHO also provides for adjusting the thresholds for people living at altitude and for smokers, in whom hemoglobin is physiologically higher. That is, the same figure in a mountain dweller and in a person from a lowland area may have a different meaning.
In endurance athletes, because of increased plasma volume, hemoglobin may be somewhat lower than in untrained people, while the total mass of hemoglobin in the body is on the contrary higher. That is why a «borderline» result in an athlete is assessed by the doctor together with ferritin, red cell indices and how the person feels.
Any pronounced decrease in hemoglobin requires a search for the cause: deficiency of iron, vitamin B12 or folic acid, hidden bleeding, chronic diseases. Taking iron-containing supplements on your own without an examination can mask a problem, for example bleeding from the digestive tract.
Editorial conclusions
The hemoglobin test is simple, but its accuracy depends on the details: venous blood is more reliable than capillary, the usual drinking regimen is more reliable than «loading» with water, and a break from training helps to assess the true baseline level.
Tell the doctor about your menstrual cycle, a recent blood donation, iron intake, hormonal drugs and whether you smoke or have recently been in the mountains. All of this changes the interpretation.
A low hemoglobin is not a diagnosis but a reason to determine the cause. Treatment is selected by the doctor after additional tests.
We also recommend reading our materials on preparing for the hematocrit test, on ferritin and iron deficiency in athletes and on the effect of androgens on blood indicators.
References
- World Health Organization. Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. WHO/NMH/NHD/MNM/11.1. Geneva: WHO; 2011.
- World Health Organization. Guideline on haemoglobin cutoffs to define anaemia in individuals and populations. Geneva: WHO; 2024.
- Simundic AM, Bölenius K, Cadamuro J, et al. Joint EFLM-COLABIOCLI recommendation for venous blood sampling. Clin Chem Lab Med. 2018;56(12):2015–2038.
- Mairbäurl H. Red blood cells in sports: effects of exercise and training on oxygen supply by red blood cells. Front Physiol. 2013;4:332.
- World Anti-Doping Agency. World Anti-Doping Code International Standard: Prohibited List. Montreal: WADA; щорічне видання.
- Rifai N, Horvath AR, Wittwer CT (eds). Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. St. Louis: Elsevier; 2018.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
