Physical properties and reaction
Normal urine is transparent, and its straw-yellow color is due to pigments called urochromes. At high concentrations, it darkens to yellow-brown, while strong dilution turns it pale yellow. Color changes are an important diagnostic sign:
- Pale urine: characteristic of chronic kidney disease (CKD), as the kidneys stop excreting urochromes.
- Dark brown: indicates active protein breakdown in the body (febrile states, hyperthyroidism, tumor processes) or the intake of certain medications (metronidazole).
- «Meat slops» appearance: a specific appearance caused by a combination of mucus, leukocytes, and gross hematuria.
Cloudiness of the fluid is associated with the presence of formed elements (bacteria, leukocytes) or salts. Urates form an orange-red sediment, while phosphates form a red-brown sediment. The pH reaction is normally predominantly acidic, although a plant-based diet causes alkalinization.
Chemical composition: protein and other metabolites
The detection of protein (proteinuria) is a key symptom of kidney damage. Qualitative reactions detect it at concentrations above 0.033 g/L. The biuret test, based on colorimetry, is considered the most accurate quantitative method.
Proteinuria does not always indicate disease. Functional proteinuria may occur with prolonged standing (orthostatic), fever, or intense physical exertion (march proteinuria). Of particular importance are:
- Microalbuminuria (25–200 mcg/min): a marker of the preclinical stage of kidney damage, for example, in diabetic nephropathy.
- Paraproteinemia: excretion of immunoglobulin light chains (determined by the Bence-Jones protein test), which is characteristic of multiple myeloma.
Other important parameters: glucose is normally practically not excreted (up to 0.3 g/day), and ketone bodies appear only during starvation. Bilirubin is filtered into the urine exclusively as the conjugated (direct) fraction in pathologies.
Urinary sediment microscopy
For the analysis, sediment is obtained by centrifuging the urine. Cells, casts, and crystals are analyzed in the sediment.
- Erythrocytes: the presence of more than 2 cells per high-power field is considered pathological (hematuria). Phase-contrast microscopy is used to differentiate dysmorphic and isomorphic erythrocytes. It should be remembered that ascorbic acid intake can cause false-negative results on test strips.
- Leukocytes: normal count is up to 5 cells. Predominance of neutrophils indicates infection, while lymphocytes indicate immune-mediated inflammation. Pronounced leukocyturia visible to the naked eye is called pyuria.
- Epithelium: squamous, transitional, and renal epithelial cells can be found in the sediment. The appearance of atypical cells requires ruling out malignancy of the urinary tract.
- Casts: protein molds of the tubules. Hyaline casts consist solely of protein and can be found in healthy individuals. Granular and waxy casts reliably indicate renal parenchyma damage.
- Crystals: depend on pH. In an acidic reaction, uric acid and cystine crystals precipitate, while in an alkaline reaction, calcium phosphates and oxalates precipitate.
Quantitative diagnostic methods
Unlike the general urinalysis, quantitative tests are standardized by collection time or fluid volume, allowing for a more precise assessment of the degree of cellular sediment:
- Nechiporenko test: counting formed elements in 1 mL. Normal range: erythrocytes up to 1,000, leukocytes up to 2,000.
- Kakovsky-Addis test: urine is collected over 24 hours. Normal range: erythrocytes up to 1 million, leukocytes up to 2 million.
- Amburgey's method: evaluates the number of elements excreted by the kidneys per minute.
Briefly: answers to topical questions
What pathologies cause the appearance of waxy casts in urinary sediment?
The appearance of waxy casts in urinary sediment indicates damage to the renal parenchyma.
In the composition-based classification, waxy casts belong to the following group:
- Pure protein casts — hyaline casts also belong to this category.
The presence of such elements (along with granular casts) is an important diagnostic marker, although specific nosological forms of renal pathologies are not listed in the sources.
What are the causes of ketone bodies in urine, aside from starvation?
Aside from starvation, ketone bodies in urine (ketonuria) are indicated in sources for diabetic ketoacidosis and ketoacidotic coma.
Conditions in which ketonuria is described include:
- Diabetic ketoacidosis — laboratory criteria include a sharp rise in blood glucose, ketonemia, ketonuria, and metabolic acidosis.
- Ketoacidotic coma in diabetes mellitus — ketonuria is listed among the laboratory parameters.
In healthy individuals, ketone bodies appear only against the background of starvation.
What are the normal reference ranges for erythrocytes, leukocytes, and casts in the Nechiporenko test?
Normal values for the Nechiporenko test are given for erythrocytes and leukocytes, while standards for cast content are not specified.
The method involves counting elements in 1 mL of urine with the following normal ranges:
- Erythrocytes — up to 1,000 per 1 mL.
- Leukocytes — up to 2,000 per 1 mL.
This study standardizes the quantitative assessment of urinary sediment by a specific fluid volume, but norms for casts using this method are absent in the sources.
Class and exam questions
What does a decrease in proteinuria during prolonged kidney disease mean? Is this a sign of recovery?
Not always. In the development of chronic kidney disease (CKD), a decrease in protein is associated with tissue sclerosis and a reduction in the number of functioning nephrons, indicating a worsening condition.
Why is it important to use phase-contrast microscopy in hematuria?
This method makes it possible to distinguish altered (dysmorphic) erythrocytes from unchanged ones. This is necessary for the differential diagnosis of the level of urinary tract lesions.
Does the presence of bacteria in urine always require treatment?
The criterion for true bacteriuria is a concentration of more than 100,000 (10⁵) bacteria per 1 mL. A urine culture is required to clarify the diagnosis and prescribe antibiotics.
What does the appearance of hyaline casts in the analysis indicate?
Hyaline casts (pure protein) can form in completely healthy people, for example, after physical exertion, and therefore have low diagnostic value on their own.
What else to study on this topic
- Pathophysiology of nephrotic syndrome development
- Specifics of detecting Bence-Jones protein and diagnosing multiple myeloma
- Topical diagnosis of renal tract lesions based on cellular sediment
- Staining of sediment smears: Romanowsky-Giemsa and Gram stains
- False and true hematuria: differential diagnosis
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