How to prepare for the 'Urea' test: rules for taking it

Urea is the end product of protein metabolism, formed in the liver and excreted by the kidneys. The urea test is part of a basic biochemical examination and, together with creatinine, is used to assess kidney function. For athletes on a high-protein diet, this indicator often turns out to be «above the norm» — and not always because of kidney problems. The editorial team explains how to prepare properly and what the result actually shows.

Where urea comes from

When the body breaks down proteins — from food or its own tissues — nitrogen in the form of ammonia is released from the amino acids. Ammonia is toxic, so the liver converts it into urea in the ornithine cycle. Urea enters the blood, is filtered by the kidneys and excreted in the urine.

From this scheme follows the main point: the concentration of urea in the blood depends not only on kidney function but also on how much urea is formed. More protein in the diet, increased breakdown of the body's own proteins, bleeding in the gastrointestinal tract — and there is more urea even with healthy kidneys.

In addition, part of the urea is reabsorbed back into the blood in the kidneys. This proportion increases with dehydration and reduced blood flow in the kidneys. Therefore urea is a sensitive but low-specificity indicator, and it is always assessed in context.

In Ukrainian laboratories the result is usually reported in mmol/L of urea. In English-language sources, the indicator BUN — blood urea nitrogen in mg/dL — is often used. These are different ways of expression, so when comparing data from foreign articles, the units need to be converted.

Diet and protein before the test

The amount of protein in the diet is one of the strongest factors affecting the urea level. Athletes who consume a lot of protein from food and protein supplements naturally have higher values. This reflects a larger volume of nitrogen that needs to be excreted, not kidney damage.

Studies examining high-protein diets in athletes found no signs of impaired kidney function in healthy people, although the urea level and nitrogen excretion were higher. The position of the International Society of Sports Nutrition also notes that in healthy people high protein intake is not associated with kidney damage, but people with kidney diseases need medical supervision.

For preparation, not only the overall diet is important but also the last meal. A large steak or several servings of protein in the evening can raise urea in the morning. Therefore, for 1–2 days before the test it is advisable to follow your usual diet without «protein overloads», and dinner should be moderate.

The opposite situation is a very low-protein diet, prolonged fasting or a vegan diet with a small amount of protein. In such people urea may be below the reference range. This also needs to be taken into account when comparing results over time.

Water, training and medications

Dehydration raises urea. With a lack of fluid, renal blood flow decreases, and urea reabsorption increases. Athletes after cutting, «making weight» before competitions, a sauna or training in the heat can get a noticeably elevated result. Before the test it is important to drink your usual amount of water.

Intense and prolonged physical exertion increases the breakdown of proteins and can temporarily raise urea. This especially applies to many-hour endurance workouts and competitions. For a stable result, for 1–2 days before the test it is better to limit yourself to light activity.

Medications also matter. Glucocorticoids increase the catabolism of proteins and can raise urea. Some antibiotics, diuretics and drugs affecting renal blood flow, for example high-dose non-steroidal anti-inflammatory drugs, also change the indicator. Prescribed drugs must not be discontinued on your own.

Anabolic steroids, on the contrary, have an anticatabolic effect and can theoretically reduce urea formation, but at the same time such drugs can negatively affect the kidneys and other organs. The doctor should know about any hormonal drugs in order to interpret the biochemistry correctly.

Rules for taking the test

Preparation for the urea test is not complicated, but it has a few important points. A short list from the editorial team:

1. draw blood in the morning, after 8–12 hours without food, water permitted;
2. 1–2 days before — usual diet, without excessive amounts of meat and protein shakes in the evening;
3. drink your usual amount of water, do not become dehydrated and do not «flood» yourself with fluid;
4. 24–48 hours before — no exhausting workouts and no sauna;
5. 24 hours before — no alcohol;
6. inform the doctor about medications, hormones and supplements.

Excessive drinking right before the test is also undesirable: it can dilute the blood and somewhat lower the indicator. The goal is a normal water balance that is typical for you.

Urea is almost always taken together with creatinine, and often with other biochemical indicators: electrolytes, glucose, liver enzymes. Therefore preparation must also take into account the requirements for them, in particular fasting.

For tracking trends, for example during a change of diet or a training block, take the test under identical conditions and at one laboratory. This makes it easier to distinguish the effect of diet from real changes in kidney function.

How to interpret it together with creatinine

Urea on its own is not an accurate marker of kidney function. To assess filtration, the KDIGO guidelines recommend using creatinine with calculation of the glomerular filtration rate, and in doubtful cases — cystatin C. Urea supplements the picture but does not replace these indicators.

The ratio of urea to creatinine is useful. If urea is elevated disproportionately to creatinine, the cause is more often «extrarenal» factors: dehydration, a high-protein diet, catabolism, bleeding in the GI tract. But if both indicators are elevated, the doctor considers the possibility of a decrease in kidney function.

In athletes with a large muscle mass, creatinine can also be higher than the reference range, since it is formed from muscle creatine. Taking creatine as a supplement also raises creatinine. Therefore the combination of «high protein + large muscle mass + creatine» often produces a picture that requires careful interpretation by a doctor and, if necessary, determination of cystatin C.

Low urea is rarely a problem, but in combination with other changes it may indicate a disturbance of liver function or inadequate nutrition. Any persistent deviations should be discussed with a doctor.

Editorial conclusions

Urea depends on protein intake, water balance, training and medications no less than on kidney function.

For an informative test, draw blood in the morning while fasting, after 1–2 days of your usual diet without protein overloads, without dehydration and exhausting exertion.

Urea should be interpreted only together with creatinine and GFR, and in athletes with a large muscle mass — with cystatin C if necessary.

We also recommend reading our materials on preparing for the creatine phosphokinase test, on fasting glucose, and on cortisol.

References

1. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl. 2013;3(1):1–150.
2. Poortmans JR, Dellalieux O. Do regular high protein diets have potential health risks on kidney function in athletes? Int J Sport Nutr Exerc Metab. 2000;10(1):28–38.
3. Martin WF, Armstrong LE, Rodriguez NR. Dietary protein intake and renal function. Nutr Metab (Lond). 2005;2:25.
4. Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition position stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20.
5. Rifai N, Horvath AR, Wittwer CT (eds). Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018.