Examination: The sediment is first examined under low power to identify most crystals, casts, squamous cells, and other large objects. The numbers of casts seen are usually reported as number of each type found per low power field (LPF). Example: 5-10 hyaline casts/L casts/LPF. Next, examination is carried out at high power to identify crystals, cells, and bacteria. The various types of cells are usually described as the number of each type found per average high power field (HPF). Example: 1-5 WBC/HPF.
Red Blood Cells: Hematuria is the presence of abnormal numbers of red cells in urine due to: glomerular damage, tumors which erode the urinary tract anywhere along its length, kidney trauma, urinary tract stones, renal infarcts, acute tubular necrosis, upper and lower urinary tract infections, nephrotoxins, and physical stress. Red cells may also contaminate the urine from the vagina in menstruating women or from trauma produced by bladder catheterization. Theoretically, no red cells should be found, but some find their way into the urine even in very healthy individuals.
White Blood Cells: Pyuria refers to the presence of abnormal numbers of leukocytes that may appear with infection in either the upper or lower urinary tract or with acute glomerulonephritis. Usually, the WBC's are granulocytes. White cells from the vagina, especially in the presence of vaginal and cervical infections, or the external urethral meatus in men and women may contaminate the urine.
Squamous epithelial cells from the skin surface or from the outer urethra can appear in urine. Their significance is that they represent possible contamination of the specimen with skin flora.
Casts: Urinary casts are formed only in the distal convoluted tubule (DCT) or the collecting duct (distal nephron). The proximal convoluted tubule (PCT) and loop of Henle are not locations for cast formation. Hyaline casts are composed primarily of uromodulin (Tamm-Horsfall) protein secreted by tubule cells. A small number of hyaline casts with this protein matrix may be found in persons with no disease.
Red blood cells may stick together and form red blood cell casts. Such casts are indicative of glomerulonephritis, with leakage of RBC's from glomeruli, or severe tubular damage.
White blood cell casts are most typical for acute pyelonephritis, but they may also be present with glomerulonephritis. Their presence indicates inflammation of the kidney, because such casts will not form except in the kidney.
When cellular casts remain in the nephron for some time before they are flushed into the bladder urine, the cells may degenerate to become a coarsely granular cast, later a finely granular cast, and ultimately, a waxy cast. Broad casts are most likely to be seen in end-stage chronic renal disease.
In end-stage kidney disease of any cause, the urinary sediment often becomes very scant because few remaining nephrons produce dilute urine.
Bacteria: Bacteria are common in urine specimens because of the abundant normal microbial flora of the vagina or external urethral meatus and because of their ability to rapidly multiply in urine standing at room temperature.
Therefore, microbial organisms found in all but the most scrupulously collected urines should be interpreted in view of clinical symptoms. Only rods are visible without staining.
Diagnosis of clinically significant bacteriuria in a case of suspected urinary tract infection requires culture.
A colony count may also be done to see if significant numbers of bacteria are present. Generally, more than 100,000/ml of one organism reflects significant bacteriuria. Multiple organisms reflect contamination. However, the presence of any organism in catheterized or suprapubic tap specimens should be considered significant.
Yeast: Yeast cells may be contaminants or represent a true yeast infection. Most often they are Candida, which may colonize bladder, urethra, or vagina.
Crystals: Common crystals include calcium oxalate, triple phosphate crystals and amorphous phosphates. In small numbers they may be seen even in healthy patients.
Oxalate crystals are often present in small numbers in normal urine. They can be numerous in ethylene glycol poisoning.
Triple phosphate (magnesium ammonium phosphate) crystals may be present with urinary tract infection caused by bacterial organisms containing urease.
Urate crystals occur in large numbers in acute urate nephropathy (for example tumor lysis syndrome). They may also be present in normal urine at acid pH.