MICROSCOPIC URINALYSIS

MICROSCOPIC URINALYSIS

 

Methodology

A sample of well-mixed  urine (usually 10-15 ml) is centrifuged in a test tube at relatively low speed (about 2-3,000 rpm) for 5-10 minutes until a moderately cohesive button is produced at the bottom of the tube. The supernatant is decanted and a volume of 0.2 to 0.5 ml is left inside the tube. The sediment is resuspended in the remaining supernatant by flicking the bottom of the tube several times. A drop of resuspended sediment is poured onto a glass slide and coverslipped.

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. Since the number of elements found in each field may vary considerably from one field to another, several fields are averaged. 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 uri 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 catherization. Theoretically, no red cells should be found, but some find their way into the urine  even in very healthy individuals. However, if one or more red cells can be found in every high power field, and if contamination can be ruled out, the specimen is probably abnormal.

RBC's may appear normally shaped, swollen by dilute urine  (in fact, only cell ghosts and free hemoglobin may remain), or crenated by concentrated urine . Both swollen, partly hemolyzed RBC's and crenated RBC's are sometimes difficult to distinguish from WBC's in the urine . In addition, red cell ghosts may simulate yeast. The presence of dysmorphic RBC's in  urine suggests a glomerular disease such as a glomerulonephritis. Dysmorphic RBC's have odd shapes as a consequence of being distorted via passage through the abnormal glomerular structure.

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 .

If two or more leukocytes per each high power field appear in non-contaminated urine , the specimen is probably abnormal. Leukocytes have lobed nuclei and granular cytoplasm.

Epithelial Cells

Renal tubular epithelial cells, usually larger than granulocytes, contain a large round or oval nucleus and normally slough into the urine  in small numbers. However, with nephrotic syndrome and in conditions leading to tubular degeneration, the number sloughed is increased.

When lipiduria occurs, these cells contain endogenous fats. When filled with numerous fat droplets, such cells are called oval fat bodies. Oval fat bodies exhibit a "Maltese cross" configuration by polarized light microscopy.

Transitional epithelial cells from the renal pelvis, ureter, or bladder have more regular cell borders, larger nuclei, and smaller overall size than squamous epithelium. Renal tubular epithelial cells are smaller and rounder than transitional epithelium, and their nucleus occupies more of the total cell volume.

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 a mucoprotein (Tamm-Horsfall protein) secreted by tubule cells. The Tamm-Horsfall protein secretion (green dots) is illustrated in the diagram below, forming a hyaline cast in the collecting duct:

Even with glomerular injury causing increased glomerular permeability to plasma proteins with resulting proteinuria, most matrix or "glue" that cements urinary casts together is Tamm-Horsfall mucoprotein, although albumin and some globulins are also incorporated. An example of glomerular inflammation with leakage of RBC's to produce a red blood cell cast is shown in the diagram below:

The factors which favor protein cast formation are low flow rate, high salt concentration, and low pH, all of which favor protein denaturation and precipitation, particularly that of the Tamm-Horsfall protein. Protein casts with long, thin tails formed at the junction of Henle's loop and the distal convoluted tubule are called cylindroids. Hyaline casts can be seen even in healthy patients.

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. Granular and waxy casts are be believed to derive from renal tubular cell casts. Broad casts are believed to emanate from damaged and dilated tubules and are therefore seen in end-stage chronic renal disease.

                                                                  granular cast

                                                                 Waxy Cast

The so-called telescoped urinary sediment is one in which red cells, white cells, oval fat bodies, and all types of casts are found in more or less equal profusion. The conditions which may lead to telescoped sediment are: 1) lupus nephritis 2) malignant hypertension 3) diabetic glomerulosclerosis, and 4) rapidly progressive glomerulonephritis.

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.

Diagnosis of 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. They are often difficult to distinguish from red cells and amorphous crystals but are distinguished by their tendency to bud. Most often they are Candida, which may colonize bladder, urethra, or vagina.

                                                                    Yeast

Crystals

Common crystals seen even in healthy patients include calcium oxalate, triple phosphate crystals and amorphous phosphates.

Very uncommon crystals include: cystine crystals in of neonates with congenital  urine cystinuria or severe liver disease, tyrosine crystals with congenital tyrosinosis or marked liver impairment, or leucine crystals in patients with severe liver disease or with maple syrup  urine disease.

Miscellaneous

General "crud" or unidentifiable objects may find their way into a specimen, particularly those that patients bring from home.

Spermatozoa can sometimes be seen. Rarely, pinworm ova may contaminate the urine  In Egypt, ova from bladder infestations with schistosomiasis may be seen.