Basic parameters and screening
To quickly assess the overall function of the organ, specific gravity in a single urine portion and a blood biochemistry test for creatinine are used.
Serum creatinine level is the most reliable marker of functional status, as it directly depends on filtration efficiency. In turn, the specific gravity of the morning portion allows for rapid screening. If this value exceeds 1.020 (or at least 1.018), the concentration capacity is considered good. With such results, ordering additional, more complex functional tests is usually not required.
Zemitsky test: methodology and norms
This is the most common functional test, proposed by S.S. Zemitsky. Its physiological purpose is to test the ability of the kidneys to concentrate urine under a standard fluid regimen (fluid intake is restricted to 1500 ml per day).
The patient urinates spontaneously, collecting the material into exactly 8 containers — one for every 3 hours over the course of a day. The volume and specific gravity of each obtained portion are measured in the laboratory.
Physiological norm:
- Pronounced variation in specific gravity values throughout the day.
- The maximum specific gravity in at least one of the portions rises above 1.018.
- The volume of individual portions ranges from 50 to 250 ml.
- Daytime diuresis accounts for 2/3 to 3/4 of the total volume of excreted urine, significantly exceeding nighttime diuresis.
Abnormalities in the Zemitsky test
Analysis of 24-hour urine portions allows for the detection of severe abnormalities in the urinary system.
- Nycturia: a condition in which nighttime diuresis exceeds daytime diuresis. This is a clear sign of functional insufficiency. The kidneys are working less effectively, so they require extra time during the night hours to excrete the metabolic products accumulated during the day.
- Minor abnormalities: narrowing of the specific gravity oscillation amplitude (e.g., in the range of 1.009–1.016). Indicates initial dysfunction.
- Isosthenuria: a key and formidable sign of renal failure (formerly termed asthenuria). It manifests as a rigid fixation of specific gravity within the narrow limits of 1.008–1.010 or the inability to exceed the 1.012 threshold. Urine osmolarity thus compares to the osmolarity protein-free plasma filtrate (isos — equal). This indicates a complete loss of concentrating function, which is characteristic of irreversible renal shrinkage. The patient excretes watery, pale, odorless urine.
Loading tests
Provocation tests are used for an in-depth study of functional reserves.
Concentration tests:
- Dry diet test (fluid deprivation): complete absence of drinking for 36 hours. Normally, specific gravity should rise to 1.022–1.040.
- Vasopressin test: after 18–24 hours without water, the patient is injected with 5 IU of vasopressin. In 90% of healthy individuals, the specific gravity reaches 1.023.
- Reserve assessment: stimulation of hyperfiltration using a protein load or dopamine administration.
Dilution test: Water load is administered (the patient drinks water at 20 ml per kilogram of body weight). Evaluation is carried out after 4 hours. Healthy kidneys should excrete more than 80% of the ingested volume, and urine specific gravity should briefly drop to 1.001–1.002.
Acid-excreting function
In a baseline fasting state, normal urine pH is maintained below 5.5. To test the ability of the kidneys to regulate acid-base balance, an ammonium chloride loading test is used (dose 100 mg/kg).
The mechanism of action is based on the fact that ammonium chloride artificially reduces plasma bicarbonate concentration (less than 20 mEq/L). The normal reaction of healthy kidneys to such a shift is active urinary acidification, in which urine pH should drop below 5.5.
Briefly: answers to topical questions
What formulas are used to calculate the glomerular filtration rate (GFR)?
Glomerular filtration rate is calculated using validated formulas that take into account individual patient parameters.
- Cockcroft-Gault formula — uses age, body mass, and serum creatinine level; for women, the result is additionally multiplied by a factor of 0.85.
- CKD-EPI — is the preferred formula for adult patients, as it shows the best correlation with reference clearance methods (pregnancy is an exception).
- Schwartz formula (Bedside modification) — used in children; calculation is performed by multiplying 36.2 by height in centimeters and dividing by plasma creatinine level.
What is hyposthenuria and in what diseases does it occur?
Hyposthenuria is a decrease in urine specific gravity below normal (less than 1.009) caused by impaired renal concentrating function.
According to the provided sources, it is indicated in the following conditions:
- Leptospirosis — during recovery, oligo-/anuria is replaced by polyuria with hyposthenuria, after which concentrating function recovers within 2–4 weeks.
- Pyelonephritis — in a chronic process, polyuria with hyposthenuria is possible against the background of decreased renal concentrating ability.
Class and exam questions
What is the essence of isosthenuria and why is it dangerous?
Isosthenuria is a condition where urine specific gravity is fixed at 1.008–1.012, which equals plasma density. This means the kidneys have completely lost the ability to concentrate urine, which is a sign of pronounced and often irreversible damage (shrunken kidney).
How to quickly ensure that renal function is normal without complex tests?
It is sufficient to measure the specific gravity of a single morning portion of urine. If the indicator exceeds 1.018 (and according to some data — 1.020), overall renal function is considered good.
What does the predominance of nocturnal diuresis in the Zemitsky test indicate?
This condition is called nycturia. It indicates that due to reduced function, the kidneys do not have time to excrete metabolic products during the day and are forced to continue active work at night.
How is the kidney's ability to dilute urine tested?
A water-load test is used: the patient drinks water (20 ml/kg of body weight). After 4 hours, healthy kidneys should excrete more than 80% of the ingested volume, and urine specific gravity should decrease to 1.001–1.002.
What else to study on this topic
- Calculation of glomerular filtration rate (GFR) by creatinine level
- Pathophysiology of the loss of concentrating ability
- Mechanism of action of vasopressin in the renal tubules
- Assessment of functional renal reserve (protein and dopamine tests)
- Role of plasma bicarbonates in acid-base balance regulation
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