Renal functional tests

Main functional and quantitative studies

General clinical analyses establish the presence of protein or cells in urine, but specialized tests are required to accurately understand the functional state of the kidneys:

• Rehberg's test is used to study the excretory function of nephrons. It is based on calculating the glomerular filtration rate by evaluating creatinine clearance.
• Zimnitsky's test is used to analyze the concentrating capacity of the kidneys. The method assesses fluctuations in the specific gravity of urine across various portions collected throughout the day.
• Daily proteinuria is essential for the quantitative determination of protein lost by the body over a 24-hour period. This test is critical for detailing a comprehensive urinary syndrome.
• Nechyporenko test serves as a confirmatory method. It is ordered if a routine urinalysis reveals a threshold leukocyte count (≥7 per high-power field). The method allows for the quantitative evaluation of formed elements in 1 milliliter of urine.

Differential diagnosis of urinary syndrome

A decrease or alteration in renal function clinically manifests as various variants of urinary syndrome. Evaluating its leading component allows for structuring a diagnostic algorithm:

1. Isolated hematuria (without edema, hypertension, or significant proteinuria). Indicates urological pathology (trauma, stones, tumors), IgA nephropathy, or thin basement membrane disease.
2. Nephritic syndrome. The classic triad includes hematuria, arterial hypertension, and edema (plus proteinuria). It is characteristic of acute glomerulonephritis and carries a high risk of developing acute kidney injury.
3. Nephrotic syndrome. Manifests as massive proteinuria, hypoalbuminemia, hyperlipidemia, and pronounced edema. Arterial hypertension and hematuria are not prominent (found in membranous glomerulopathy, minimal change disease).
4. Tubulointerstitial damage. Manifests with predominant bacteriuria and neutrophilic leukocyturia (typical for pyelonephritis).
5. Immune glomerular inflammation. The marker is abacterial lymphocytic leukocyturia.

Dynamics of renal function in infectious processes

Renal tests can change in the setting of extrarenal diseases and systemic infections. Changes can be true (nephrological) or transient (reactive).

A striking example of cyclical changes in renal function is hemorrhagic fever with renal syndrome (HFRS):

• Febrile period: urine specific gravity increases, moderate proteinuria and fresh erythrocytes appear.
• Oliguric period: massive proteinuria, gross and microhematuria, and cylindruria develop. Degenerated renal epithelium (so-called Dunaevsky cells) is found in the sediment.
• Polyuric period: hyposthenuria (low urine density) and nocturia occur. This is associated with the slow recovery of reabsorptive function and tubular epithelium.

With marked intoxication in the setting of measles or ARVI, reactive proteinuria, cylindruria, leukocyturia, and an increase in urine density are also possible. These shifts disappear after the body temperature normalizes. It is important to differentiate such conditions from true pathology. For instance, in alcoholic cirrhosis of the liver, changes in urine help rule out hepatorenal syndrome and diagnose primary renal damage.

Functional assessment of urine absence

A sharp decrease or complete cessation of urine output requires immediate differential diagnosis between two fundamentally different conditions:

• True anuria. The kidneys stop producing urine. Urine does not enter the bladder. This occurs due to impaired renal blood flow (shock of any etiology), nephrotoxic damage, or acute parenchymal inflammation.
• Acute urinary retention (ischuria). The kidneys function and produce urine normally, but the process of its elimination from the bladder is impaired. Causes may include prostate pathology, central nervous system diseases, paraproctitis, or iatrogenic factors (use of ganglion blockers, atropine, narcotic analgesics). The main diagnostic sign is a significantly enlarged, overfilled bladder determined by percussion.

Class and exam questions

What else to study on this topic

• General clinical and biochemical urine parameters (glucose, urea, electrolytes)
• Clinical manifestations and pathogenesis of nephritic and nephrotic syndromes
• Diagnostics and surgical treatment of ureterocolic and urethrovaginal fistulas
• Microscopic examination of urine sediment and identification of cylinder types

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.