Urinalysis

The Urinalysis, Urinalysis, is a basic diagnostic tool and valuable for any medical specialty. The urinalysis allows direct conclusions to be drawn about the general state of health of a patient, in particular the state of the kidneys and the lower urinary tract.

What is the urinalysis?

Urine is a sterile blood serum concentrate. The urine is produced by the filter function of the kidneys as organs arranged in retroperitoneal pairs. The kidneys constantly filter the blood and rid it of toxins or metabolic products.

In the course of 24 hours, around 1,500 liters of primary urine are concentrated and dehydrated to produce around 1.5 liters of terminal urine, which gets from the kidneys through the ureters into the bladder. If enough terminal urine has accumulated in the bladder, a reflex of the bladder muscles empties urine, micturition via the urethra. This urine is then used as so-called mid-stream urine for examination purposes.

Modern urine analysis using multiple test strips and microscopic, cytological and pathological examinations goes back to the traditional urine inspection of the Middle Ages. At that time, the chemical composition of urine was not yet known in detail. The color, smell and taste of the urine played an important role in diagnosing diseases. For example, sweet tasting urine indicated diabetes mellitus. Today, glucose detection takes place via a chemical reaction on the field of a urine test strip, which is dipped into the urine by the doctor or medical assistant.

Function, effect & goals

The color of the urine depends mainly on the fluid intake but also on different foods. For example, beetroot stains the urine red. A red colouration can also be caused by the presence of blood in the urine, a pathological finding that always needs to be clarified and which mostly indicates an inflammation of the kidneys or the urinary tract.

The macroscopic urine examination, as it was traditionally practiced as urine examination in the Middle Ages, is no longer common today. In addition to fluid intake, the appearance of the urine can also be significantly changed by medication or internal diseases and inflammation. Significantly different changes in appearance are documented if they are relevant to the diagnosis. A urinalysis, like a blood test or anamnesis, is one of the usual examinations as part of a general practitioner's consultation.

Doctors of any specialty can interpret urine findings, but the urologist or nephrologist is particularly qualified to interpret findings or to carry out an extended urine analysis for special medical questions. To determine the specific urine weight, a measuring spindle is immersed in the urine, which precisely determines the weight-volume ratio. It is therefore possible to make a statement about how many dissolved components are contained in the urine sample. Concentrated urine has a higher specific gravity because it contains less water.

The standard urinalysis is carried out using multiple test strips, the so-called Combur test. Up to 10 different test fields are available on such a test implement. The urine test using test strips is carried out in three steps: immersion, wiping off and reading.

The test results can therefore be read off and documented by visual color comparison immediately after immersion with a color scale. Important test fields are glucose, pH value, blood, hemoglobin, leukocytes, nitrite, protein and urobilinogen. In the test fields, the substances dissolved in the urine lead to a complicated chemical reaction that is enzymatically linked to a typical color change. For example, the yellowish initial test field turns green when blood is present in the urine; the white test field turns pink when it comes into contact with leukocytes in the urine.

The intensity of the discoloration is a measure of the concentration of the pathological substance. If the urine contains bacteria due to an inflammatory process, the nitrite test field changes color, provided that it is nitrite-forming bacteria, which are responsible for over 90 percent of all urinary tract infections. The examination with the test strip can be followed by a microscopic analysis of the so-called urine sediment. The urine sediment is collected by centrifuging the urine.

Due to the centrifugal force, the cellular components of the urine accumulate at the bottom of the glass tube. The excess urine is decanted. Bacteria, epithelial cells, cylinders or crystals can be analyzed in the urine sediment. Bacteria can also be cultivated from the sediment with the aid of a nutrient medium for the final diagnosis and determination of resistance.

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Risks, side effects & dangers

Urine is an important diagnostic tool for the initial diagnosis, for suspected diagnoses, exclusion diagnoses or for monitoring the progress of internal diseases and diseases of the lower urinary tract. The urine used for the analysis must be as fresh as possible, because urine begins to decompose due to its biological composition just a few minutes after excretion. Only so-called midstream urine should be used for urinalysis.

When giving urine, the patient is encouraged to first discard the first portion of urine, then fill the central stream into the cup and discard the last third. Failure to comply can result in false positives. In order to standardize the test strip analysis of the urine, automatic reading machines are now also used in large practices and laboratories. This achieves far better results than manual reading. If the results are unclear or doubtful, the urine can also be obtained directly from the urinary bladder by a suprapubic bladder puncture under sterile conditions.