The kidneys are the body’s main filter, cleaning the blood of toxins and excess fluid. To check the functioning of these organs, doctors prescribe kidney tests , an analysis that consists of three main markers:
- Creatinine is a breakdown product of muscle protein that is constantly produced in the body and is normally excreted by the kidneys. A creatinine test is one of the most informative indicators of kidney function. The worse the kidneys perform their function, the more creatinine remains in the blood.
- Urea is the end product of the breakdown of proteins from food. Urea levels change not only when kidney function deteriorates. It can temporarily increase even in a healthy person with excessive meat consumption or stress.
- Uric acid is a product of the breakdown of purines. These substances, which are found in meat, fish, and beer, accumulate in the blood and joints when excreted improperly, which can cause gout.
When the kidneys are working properly, creatinine, uric acid, and urea are excreted in the urine. If there are malfunctions, they accumulate in the blood, which is reflected in the test results.
When a doctor refers for this analysis: 5 main situations
You should take a kidney test if:
Impaired kidney function, which is accompanied by swelling of the face, hands or feet, especially in the morning, or a change in the color, smell or amount of urine. Swelling is one of the first signals that indicates fluid retention. If you wake up with “bags” under your eyes, it is worth checking your kidney function. Also, darkening or colorless urine, the appearance of a pink or reddish tint, excessive foaminess, too frequent (especially at night) or very rare urination should alert you.
Urolithiasis, the main symptom of which is a pulling pain in the lower back. The reason for taking a uric acid test is constant discomfort, heaviness or aching pain in the lower back, which does not disappear after rest and is not associated with physical exertion.
Metabolic syndrome, which can occur with excess weight, diabetes, high blood pressure. These pathologies often lead to a decrease in the filtration function of the kidneys, fluid retention and stone formation. Evaluation of kidney tests allows for timely adjustment of therapy and prevention of serious complications.
The need to monitor treatment with drugs that are toxic to the kidneys. If you are constantly or for a long time taking painkillers (ibuprofen, diclofenac), antibiotics, diuretics or blood pressure medications, your doctor will definitely prescribe kidney tests in order to notice any impairment of the kidney's filtering function in a timely manner.
Preparing for surgery or CT with contrast. Contrast and most drugs used during surgery are excreted through the kidneys. Therefore, the doctor must make sure that the organs are ready for increased work.
Many kidney diseases may not cause any symptoms for a long time. That is why you should not wait for pain or swelling to appear. If your doctor recommends kidney tests or you have risk factors, this test will help to assess the condition of your kidneys in a timely manner and prevent the development of serious complications.
Norms for children and adults: why are they different?
When a kidney test is ordered, a child and an adult will have completely different reference values. This is not a mistake, but physiological characteristics. For example, the level of creatinine directly depends on muscle mass. An infant has little of it, because the kidneys are still immature, in children they are actively developing, and in adult men, greater muscle mass gives a higher level of creatinine than in women. Urea and uric acid change depending on the metabolic rate, which is very rapid in children. Therefore, the same indicator can be the norm for a 30-year-old man and a deviation for a 5-year-old child.
| Age group | Creatinine (μmol/L) | Urea (mmol/L) | Uric acid (μmol/L) |
|---|---|---|---|
| Newborns | 27–88 | 1.4–4.3 | 140–340 |
| Infants (1–12 months) | 15–35 | 1.4–5.4 | 80–240 |
| Preschoolers (1–6 years old) | 20–50 | 1.8–6.0 | 120–320 |
| Schoolchildren (7–14 years old) | 30–70 | 2.5–6.4 | 150–350 |
| Adult women | 44–97 | 2.5–8.3 | 150–350 |
| Adult men | 62–115 | 2.5–8.3 | 210–420 |
Important! Different laboratories may use different units of measurement and reference ranges. Therefore, always pay attention to the norm on the results form and discuss deviations with your doctor - he will take into account your age, gender, weight and health status.
How to prepare properly so that the result is reliable
Kidney tests are very sensitive to lifestyle choices before the lab visit. To avoid falsely high numbers, follow these rules:
Fasting before donation. The last meal should be in the evening, 8–12 hours before donating blood. In the morning, you can drink only clean, still water. Drinking tea, coffee, and juices will distort the results. Food, especially meat, increases the level of urea and uric acid.
Dietary restrictions. 1–2 days before the analysis, you should minimize the consumption of meat, fish and legumes - excess protein will artificially raise the level of urea. You should also completely abandon alcohol - it changes metabolism and blocks the excretion of uric acid.
Drinking regimen. Drink enough water. Dehydration "concentrates" the blood and can increase the indicators, even if there is no disease.
Limit physical activity. Avoid heavy physical labor and intense gym workouts during the day. Micro-tears in muscle fibers during exercise lead to a sharp jump in creatinine in the blood.
Medication coordination. Tell your doctor about all medications you are taking. Diuretics, metformin, nonsteroidal anti-inflammatory drugs, diuretics, some antibiotics, and vitamin C in large doses can change the overall picture of the analysis. If they cannot be canceled, the doctor will take into account the error in interpretation.
The result is above or below the norm - what does it mean?
Deviations from the norm in the test form are not a verdict, but only a signal from the body that requires attention and consultation with a doctor.
If the indicators are higher than normal, it means that the kidneys have slowed down their filtration work. High levels of uric acid may indicate a predisposition to gout, kidney failure or the presence of kidney stones. Elevated creatinine in a blood test is often the result of urolithiasis, inflammation or impaired blood supply to the kidneys. The concentration of urea increases with kidney disease, excessive protein intake with food and increased tissue breakdown (after surgery, injuries). But the cause of high markers can be banal dehydration (when you drank little in the heat), a recent muscle injury or an excess of steaks in the diet.
Lower than normal values are much less common. Low creatinine or urea are usually seen with very strict vegetarian diets, fasting, muscle loss due to exhaustion, or in old age. It is also possible for the markers to decrease during pregnancy, when a woman's blood volume increases, causing the values to be "diluted."
Important! If creatinine, uric acid or urea in the blood deviates from the norm, do not panic. Do not try to diagnose and prescribe treatment yourself, using the Internet. Take the form and contact your therapist or family doctor - he will evaluate the indicators in a complex and, if necessary, select effective medications.