Zimnitsky test

Currently, the most common and accessible method for determining kidney function by diluting and concentrating urine is determining the relative density of urine in a single analysis or in the Zimnitsky test.

The relative density of urine is a less sensitive indicator than osmolality. The value of the relative density of urine is affected not only by osmotically active substances, but also by high-molecular components contained in urine (proteins, sugar, contrast agents). To level out significant differences between the indicators when interpreting the data on the relative density of urine, corrections were made for the content of protein and sugar in urine. Thus, 1% sugar in urine increases the relative density of urine by 0.004; 3 g of protein - by 0.001. Accordingly, with a protein concentration in urine of 10 g / l, the value of the relative density is reduced by 0.003; with a glucose concentration of 10 g / l - by 0.0038. Determination of the relative density of urine is carried out no earlier than 3 days after studies with the introduction of contrast agents and at least 3 days after taking diuretics.

In a healthy person, the relative density in the morning portion of urine exceeding 1018 indicates the intact ability of the kidneys to concentrate urine. The ability of the kidneys not only to concentrate but also to dilute urine is determined in the Zimnitsky test. It consists of determining the relative density in eight portions of urine collected by the patient during the day at intervals of 3 hours. The test is carried out during the patient's normal physical activity, under standard drinking and eating conditions (they are allowed to drink about 1.2 liters of liquid per day) and at least 3-5 days after the withdrawal of diuretics.

In a healthy person, daily urine output is 67-75% of the amount of liquid drunk; daytime diuresis is 65-80% of the amount of urine excreted per day. Fluctuations in the relative density of urine are normally 1005-1025.

The following conditions are distinguished that characterize disturbances in the volume- and osmoregulatory functions of the kidneys based on the study of the Zimnitsky test:

• oliguria, anuria - a decrease in the amount of daily urine;
• polyuria - an excess of the amount of urine excreted over the amount of liquid drunk;
• nocturia - increased diuresis at night;
• isosthenuria - a change in the relative density of urine within the range of 1010-1011, which corresponds to the value of the relative density of blood plasma and reflects the completely impaired ability of the kidneys to concentrate and dilute urine;
• hyposthenuria - the relative density of urine in all portions is below 1012, which reflects a violation of the concentrating ability of the kidneys;
• hypersthenuria - the relative density of urine in all portions exceeds 1010, which reflects a violation of the urine dilution function.

Reference values of urine (norm) in the study according to Zimnitsky:

• daily diuresis is 0.8-2 l or 65-80% of the liquid drunk per day;
• significant fluctuations during the day in the amount of urine in individual portions (40-300 ml) and its density (1.008-1.025 g/l);
• daytime diuresis prevails over nighttime (2:1);
• the density of at least one portion is not less than 1.020-1.022 g/l.

The Zimnitsky test allows one to examine the concentration function of the kidneys. The patient remains on a normal diet, but takes into account the amount of liquid drunk. After emptying the bladder at 6 a.m., urine is collected in separate jars every 3 hours during the day, a total of 8 portions. When examining urine according to Zimnitsky, the main thing is to take into account the fluctuations in the density of individual portions of urine. If it remains at a low level, despite breaks in food and liquid intake, this indicates a violation of the kidneys' ability to concentrate urine. If the density remains at a normal level or its fluctuations do not exceed 0.007 g/l after taking liquid, this indicates a loss of the kidneys' ability to dilute.