This examination requires a degree of skill acquired through practice under the immediate supervision of an experienced technician. The specimen used for microscopic examination should be as fresh as possible. Red cells and many formed solids tend to disintegrate upon standing, particularly if the specimen is warm or alkaline.

PREPARING SPECIMENS FOR MICROSCOPIC EXAMINATION.-To prepare urine specimens for microscopic examination, follow the steps below.

1. Stir the specimen well.
2. Pour 15 ml of urine into a conical centrifuge tube, and centrifuge at 1,500 rpm for 5 minutes.

3. Invert the centrifuge tube and allow all of the excess urine to drain out. Donot shake the tube while it is inverted. Enough urine will remain in the tube to resuspend the sediment. Too much urine will cause dilution of the sediment, making an accurate reading difficult.

4. Resuspend the sediment by tapping the bottom of the tube.

5. With a medicine dropper, mount one drop of the suspension on a slide and cover it with a coverslip.

6. Place the slide under the microscope, and scan with the low-power objective and subdued lighting.

7. Switch to the high-power objective for detailed examination of a minimum of 10 to 15 fields.

CLINICALLY SIGNIFICANT FINDINGS.- Leukocytes, erythrocytes, and casts may all be of clinical significance when found in urine sediment.

Leukocytes.-Normally, 0 to 3 leukocytes per high-power field will be seen on microscopic examination. More than 3 cells per high-power field probably indicates disease somewhere in the urinary tract. Estimate the number of leukocytes present per high-power field and report it as the "estimated number per high-power field."

Erythrocytes.-Red cells are not usually present in normal urine. If erythrocytes are found, estimate their number per high-power field and report it. Erythrocytes may be differentiated from white cells in several ways:

White cells are larger than red cells.

Figure 7-22.-Index refractometer.

When focusing with the high-power lens, the red cells show a distinct circle; the white cells tend to appear granular with a visible nucleus.
One drop of 5% acetic acid added to the urine sediment disintegrates any red cells, but it does not affect the white cells (except that the nuclei become more distinct).

Casts.-These urinary sediments are formed by coagulation of albuminous material in the kidney tubules. Casts are cylindrical and vary in diameter. The sides are parallel, and the ends are usually rounded. Casts in the urine always indicate some form of kidney disorder and should always be reported. If casts are present in large numbers, the urine is almost sure to be positive for albumin.

There are seven types of casts. They are as follows:

Hyaline casts are the most frequently occurring casts in urine. Hyaline casts can be seen in even the mildest renal disease. They are colorless, homogeneous, transparent, and usually have rounded ends.

Red cell casts indicate renal hematuria. Red cell casts may appear brown to almost colorless and are usually diagnostic of glomerular disease.
White cell casts are present in renal infection and in noninfectious inflammation. The majority of white cells that appear in casts are hypersegmented neutrophils.

Granular casts almost always indicate significant renal disease. However, granular casts may be present in the urine for a short time following strenuous exercise. Granular casts that contain fine granules may appear grey or pale yellow in color. Granular casts that contain larger coarse granules are darker. These casts often appear black because of the density of the granules.

Epithelial casts are rarely seen in urine because renal disease that primarily affects the tubules is infrequent. Epithelial casts may be arranged in parallel rows or haphazardly.

Waxy casts result from the degeneration of granular casts. Waxy casts have been found in patients with severe chronic renal failure, malignant hypertension, and diabetic disease of the kidney. Waxy casts appear yellow, grey, or colorless. They frequently occur as short, broad casts, with blunt or broken ends, and often have cracked or serrated edges.

Fatty casts are seen when there is fatty degeneration of the tubular epithelium, as in degenerative tubular disease. Fatty casts also result from lupus and toxic renal poisoning. A typical fatty cast contains both large and small fat droplets. The small fat droplets are yellowish-brown in color.

SUMMARY
Clinical laboratory medicine is a very dynamic field of medicine, with new testing procedures and equipment being invented all the time. The goal of this chapter is to introduce you to some basic laboratory tests that do not require state-of-the-art equipment and that can be easily performed in isolated duty stations and aboard naval vessels. These tests will assist you in establishing diagnoses and will enable you to provide the best possible medical care for your patients.