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Urinalysis is a diagnostic tool that commonly is used in the diagnosis of urinary tract diseases and other disorders that can affect the kidneys. This is an effective way of examining kidney health because the kidneys filter urine before it is excreted. Examining the contents of urine can help provide clues about kidney health. In microscopic urinalysis, a close examination of a urine sample is made using a microscope.
Urinalysis generally involves two stages: macroscopic urinalysis and microscopic urinalysis. In macroscopic analysis, a urine sample is examined simply by looking at its color, opacity and other factors such as sugar and protein content. Healthy urine is clear, pale yellow or pale amber and does not contain particulate matter. If macroscopic analysis shows that urine is cloudy, of an abnormal color or contains a high proportion of proteins, this can help pinpoint a diagnosis.
In microscopic urinalysis, a small sample of urine is centrifuged to remove the liquid. The sediment is then examined under a microscope. In a typical microscopic urinalysis, a sample is examined for the presence of red and white blood cells, renal epithelial cells, bacteria, casts and crystals.
Small numbers of renal epithelial cells usually are present in a normal urine sample. These cells are shed naturally by the tubules of the kidneys as new cells grow. If large numbers of these cells are present in a sample, this generally is an indication of some type of disease in the kidney or upper urinary tract.
The presence of red blood cells in a urine sample can be an indication of several disease processes. These include kidney or urinary tract damage, kidney or urinary tract stones and urinary tract infection. A small amount of blood is normal in the sample of a woman who is menstruating or in the sample of someone who recently had a bladder catheter inserted. In general, red blood cells are not present in urine, and the presence of these cells almost always is an indicator of an abnormal sample.
White blood cells can be present in both normal and abnormal urine samples, but the presence of these cells likely indicates an infection. Generally, white blood cells in a urine sample are an indication of a urinary tract infection or a kidney infection. In some cases, a urine sample can be contaminated with white blood cells from the external genital area. When this happens, it often is an indication of an external genital infection.
It is common to find bacteria in a normal urine sample, particularly if the sample was given by a woman or girl. This is because a sample is likely to contain cells and bacteria from the external genital area. Bacterial microscopic urinalysis therefore involves examining the types of bacteria present in a sample, rather than the simple presence of bacteria. In most cases, this requires that a sample of the urine be cultured to determine which bacteria are present in large enough numbers to cause infection.
Casts and crystals are clumps of substances, usually cells or molecules, that are passed into the urine. Casts are congregations of red or white blood cells that have become stuck together. Generally, the presence of red blood cell casts indicates kidney infection or kidney disease of some kind. If white blood cell casts are present, this is a firm indicator of acute kidney infection or inflammation. When crystals are found in a microscopic urinalysis, this might indicate liver disease, but small numbers of casts often are seen in samples from healthy people.