What is the clinical significance of routine urine examination?

Author: Yang Qunsong, Xu Wei

Routine urine examination is an important item in health check-up. It can not only reflect urinary system diseases, but also has important significance for screening various diseases such as diabetes, icteric hepatitis, biliary obstruction, etc. It is an inexpensive and effective means of early diagnosis of diseases.

So how should we interpret it correctly?

General urine routine tests include urine color, transparency, specific gravity, urine pH value, urine white blood cells, nitrite, protein, glucose, urine ketone bodies, urobilinogen, urine red blood cells, etc.

1. Color

Normal urine color is light yellow, which may vary slightly depending on the amount of water you drink, your activities, and your diet. Common abnormal urine colors include:

01. Dark brown urine: bilirubin positive, often seen in hepatocellular jaundice, obstructive jaundice, etc.

02. Red urine: Hematuria can be seen in urinary system stones, tumors, infections (including tuberculosis), glomerulonephritis, etc.

In addition, some drugs can also cause red urine, such as aminopyrine, phenytoin sodium, rifampicin, etc. Therefore, when hematuria is found, it is necessary to first rule out the influence of these "destructive molecules", and eliminate them one by one in combination with the patient's clinical symptoms to find the cause.

03. Beer-colored to soy sauce-colored urine: hemoglobinuria, seen in favism, etc.

04. Milky urine, chyluria, and pyuria: common in filariasis, or the urine contains a large amount of inorganic salt crystals.

2. Transparency

Normal fresh urine is mostly clear and transparent. Except for urine that has been left for too long and is contaminated by female secretions, which may be slightly turbid, turbid urine is often caused by urate crystals, chyluria, pyuria, and hematuria.

3. Specific Gravity

The normal range is 1.005 to 1.030. Urine specific gravity is affected by age, water intake and sweating, and the urine specific gravity of infants and young children is relatively low.

The level of urine specific gravity mainly depends on the concentrating function of the kidneys, so it can be used as one of the renal function tests.

01. Increased specific gravity: Oliguria is seen in acute nephritis, high fever, and heart failure; polyuria is seen in diabetes;

02. Decreased specific gravity: seen in chronic glomerulonephritis, renal insufficiency, diabetes insipidus, and drinking large amounts of water.

4. pH value

The normal range is 4.6 to 8.0. The pH of urine depends largely on the type of diet, medications taken, and the type of disease.

01. Low levels are seen in diabetic ketoacidosis, hunger, severe diarrhea, respiratory acidosis, fever, etc.

02. High levels are often seen in cases of severe vomiting, persistent hyperexposure, urinary tract infection, etc.

5. White blood cells

Normal urine microscopic examination of white blood cells will not exceed 5, if there are a large number of white blood cells, the possibility of urinary tract infection should be considered. However, it should be noted that there are many factors that affect urine white blood cells:

Crystals, small round epithelial cells, yeast, Trichomonas and other components can cause an increase in white blood cells and cause a false positive.

Jaundiced urine or urine left for too long can lead to a decrease in white blood cells, causing a false negative.

Therefore, urinary tract infection cannot be judged simply from the positive urine white blood cells, but must be judged in combination with the following nitrite results.

6. Nitrite

Normally, urine nitrite is negative. Positive urine nitrite results are common in:

Pyelonephritis caused by Escherichia coli (about two-thirds), as well as urinary tract infection, cystitis, bacteriuria, etc.

The specificity of urine nitrite can reach 80%, which is of great value in assisting the diagnosis of urinary tract infection. A positive nitrite can basically be preliminarily judged as a urinary tract infection.

If the bacteria of urinary tract infection (such as positive bacteria) cannot reduce nitrate to nitrite, or the urine stays in the bladder for a short time, or there is a lack of nitrate in the urine, a negative result will also be produced, which requires attention.

7. Protein

Normal people occasionally have a small amount of protein in their urine, including albumin, globulin and some other small molecular weight proteins secreted by the renal tubules.

Normal urine routine examination is negative for urine protein, the total protein in 24-hour urine is less than 150 mg, of which albumin does not exceed 30 mg.

There are many causes of proteinuria. To diagnose pathological proteinuria, the following factors must be excluded:

01. Functional proteinuria: caused by mental stress, strenuous exercise, and low temperature stimulation, it is more common in adolescents.

02. Postural proteinuria: negative when lying down, positive when standing for too long, more common in adolescents.

03. Occasional proteinuria: Reproductive system secretions mixed into urine may cause a false positive for urine protein.

On the contrary, when the patient has been injected with large amounts of penicillin, the situation may change, leading to a false negative result for urine protein.

Urine protein test is also an important indication for patients with kidney disease. If the patient's urine protein is positive, it has clinical diagnostic value and a further 24-hour urine protein quantitative test can be performed.

8. Ketones

The qualitative test for urine ketone bodies in normal people is negative.

In the event of hunger, glucose metabolism disorders caused by various reasons, increased lipolysis and diabetic ketoacidosis, ketonemia and ketonuria may occur because the rate of ketone body production is greater than the rate of tissue utilization.

Positive urine ketone bodies are often associated with diabetes, pregnancy, malnutrition, and chronic diseases, such as diabetic ketoacidosis.

It may also be caused by severe diarrhea, vomiting, hunger, chloroform, ether anesthesia, phosphorus poisoning, taking biguanide hypoglycemic drugs, etc.

9. Urobilinogen

Urine bilirubin, urobilinogen and urobilinogen are mainly used as the basis for differential diagnosis of jaundice diseases, and their results need to be combined to guide clinical diagnosis and treatment.

This item, together with urobilinogen and urobilin, serves as the basis for differential diagnosis of jaundice diseases (the table below is for reference only).

10. Red Blood Cells

Normal urine may occasionally contain red blood cells, but no more than 3. If more red blood cells are present, they may be red blood cells discharged from the kidneys or urinary tract bleeding, and blood circulation disorders should also be considered. Urine red blood cell morphology detection is of great significance in determining the source of hematuria.

Sometimes the urine occult blood may be positive, while the urine red blood cells may be negative. This is because the red blood cells in the urine are affected by factors such as the osmotic pressure in the urine and eventually rupture.

11. Vitamin C

The significance of this indicator is not to indicate disease, but to help clinicians judge whether other test results are credible and whether they are interfered by vitamin C.

The amount of vitamin C excreted in normal urine is very small, the qualitative test is negative, and the quantitative test is generally < 0.6 mg/L (< 10 mmol per liter of urine).

If the vitamin C content in urine is high, it may affect some test items based on redox reaction, such as urine occult blood, causing false negatives.

Vitamin C is often used in clinical intravenous rehydration, which has a significant impact on urine test results, especially 30 minutes after medication, when urinary vitamin C reaches its peak, interfering with the test results.

Therefore, when testing urine routine or rechecking urine routine, try to avoid the intake of vitamin C. If the value of vitamin C in urine routine is high, you should choose an opportunity to recheck.

To sum up, a simple and easy test like urine routine test provides so much information that it is worth our good use.