Microscopic Examination of Urine Sediment
As part of a urinalysis, the urine sediment is centrifuged and examined microscopically for crystals, casts, red blood cells, white bloods cells, and bacteria or yeast. Because examination of urinary sediment provides a direct sampling of urinary tract morphology, it provides important information useful for both diagnosis and prognosis.
In healthy people, the urine contains small numbers of cells and other formed elements from the entire urinary tract, and epithelial cells from the kidney, ureter, bladder, and urethra. In renal disease, the urine often contains increased numbers of substances discharged from an organ that is otherwise accessible only by biopsy or surgery. A microscopic examination of urine sediment detects the presence and amounts of:
Urine stored in the bladder is normally free of bacteria or yeast. However, bacteria are commonly found in urine specimens because of the abundant normal microbial flora of the vagina or external urinary meatus and due to the ability of bacteria to multiply rapidly in urine standing at room temperature. Bacteria noted on a microscopic examination should be interpreted in view of clinical signs and symptoms of urinary tract infection. Diagnosis of bacteriuria in a patient with a suspected urinary tract infection requires a urine culture and sensitivity. A colony count may also be done to determine if significant numbers of bacteria are present. Generally, more than 100,000 bacteria of one organism per milliliter of urine indicates a urinary tract infection. A finding of multiple organisms usually reflects specimen contamination.
Casts are collections of protein, cells, and debris that are formed in the tubules of the kidneys. Cast width is described as narrow (one to two red blood cells in width), medium broad (three to four red cells in width), and broad (five red blood cells in width). Casts that form in the collecting tubules tend to be very broad. Broad casts usually indicate a significant reduction in the functional capacity of the nephron and indicate severe renal damage or "end stage" renal disease. A few hyaline casts are normal, but all other casts need to be evaluated. When cellular casts remain in the nephrons for some time before being flushed into the bladder urine, the cells may degenerate to a coarsely granular cast, later to a finely granular cast, and eventually, to a waxy cast. Granular and waxy casts are believed to be derived from renal tubular casts.
The number of casts are reported as "number and type seen per low power field (LPG)". An example of a report might read: "5-10 hyaline casts/LPG."
Some epithelial cells from the skin surface or from the outer urethra can appear in the urine. Some forms of crystals appear in the urine of healthy individuals. Abnormal crystals can indicate liver disease or some forms of genetic abnormalities.
After the urine sediment is examined for bacteria, casts, crystals, and epithelial cells, it is assessed under a high power field (HPF) for the presence of red and white blood cells. Normally, there should be only an occasional red blood cell in the urine (2-3 per high power field).
Hematuria, the presence of abnormal numbers of red blood cells in the urine may be due to:
In women, it is important to make sure that the urine specimen was not contaminated by the menstrual flow. Inserting a tampon and collecting urine midstream are ways to prevent contamination. Red blood cells may stick together and form red blood cell casts. The presence of oddly shaped red blood cells in the urine suggests a glomerular disease such as glomerulonephritis, as the odd shape is caused by the passage of the cells through a distorted and abnormal glomerular structure. White blood cells are normally not present in the urine. The presence of white blood cells indicates infection or inflammation within the urinary tract.
Although crystals in the urine are usually benign, urinary casts indicate renal disease.
TRUE or FALSE
The University of Utah has a great Urinalysis Tutorial
Scroll down and look at the section on MICROSCOPIC URINALYSIS. Click on the highlighted elements of the micro exam and you'll see how they look through the microscope.
(For example "red blood cells in urine" looks like this; be sure and look at the dysmorphic red cell slide also)