MICROSCOPIC EXAMINATION OF URINE

MICROSCOPIC EXAMINATION OF URINE

The most common laboratory procedure utilised for the detection of renal or urinary tract disease is the microscopic examination of urine. This can be done in various ways, but is generally carried out by taking a standard amount of urine, centrifuging the specimen for a fixed period of time, removing the supernatant fluid, resuspending the sediment in a standard volume (usually 1 ml), and examining the sediment under the microscope. 

For semiquantitative or standardised analysis of the urinary sediment, a constant volume of urine must be centrifuged and a constant volume of supernatant removed. For example, 12 ml of urine should becentrifuged and finally resuspended in 1ml. Examination of a urinary sediment enables assessment of the number of cells (white cells, red cells, epithelial cells, and tumour cells),casts, crystals and parasites in urine to be done. Each of these constituents can be a valuable indicator of pathology.

There are various ways in which a urinary sediment can be examined. With a proper level of professional expertise, bright-field microscopy of unstained urine can identify all clinically important and relevant structures in the sediment. It is necessary to adjust the amount of light entering the objective by carefully positioning the condenser and iris diaphragm. Other methods of examination include phase-contrast, interference or polarised microscopy or use of staining methods such amonochrome staining, gram staining or supra-vital staining.

Constituents of Urinary Sediment

The constituents of urinary sediment can be broadly divided into two groups: 

1. Organised sediment This represents the biological part and includes various types of cells, casts, bacteria, parasites and spermatozoa. Cells in the urine are either from the kidney or urinary tract, or from circulating blood. Organisms and tumour cells do not originally belong to the urinary system

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2. Unorganised sediment This represents the chemical portion and consists of chemical crystals and amorphous material. Their presence is generally less significant with the exception of some crystals (Fig. 4.6).Some of the prominent organised urinary sediment are as 

1.RED BLOOD CELLS

2.WHITE BLOOD CELLS

3. EPHITHILIAL CELLS

4.CASTS

5.BECTERIA AND OTHER MICRO ORGANISM

6.CRYSTALS

7.URINARY CALCULI:

Method for Microscopic Examination of Urine

For standardisation of reporting the urinary sediment is concentrated 12:1, that is 12 volumes of urine is centrifuged, supernatant removed and the sediment resuspended in 1ml of urine before examination. Any fraction of this ratio may be used (e.g., 6 ml to 0.5 ml, 3 ml to 0.25 ml)

Method

1. Pour 12 volumes (ml) of well-mixed urine in a centrifuge tube and centrifuge at 450 g (approx. 1500-2000 rpm) for 5 minutes.

2. Decant 11 volumes (ml) of urine without disturbing the sediment, leaving 1 volume (ml) in the tube.

3. Gently resuspend the sediment. Deliver one drop on a slide. Cover with a coverslip.

4. Examine the slide using low power (10 X) objective. Reduce the light intensity to minimum and scan several fields for casts. If present, report the number of cast per low power field. Identify the cast type by using high power (40 X) objective.

5. Using high power objective with slightly increased light intensity, identify and count red cells, leukocytes and renal epithelial cells. Examine at least 10 fields. Report each cell type as cells/hpf.

6. Report on

(i) Bacteria, yeast, parasites, fat globules if present as few, moderate, many or abundant. Spermatozoa should be reported if present in large numbers

(ii) Crystals: Identify and quantitate as few,moderate or many. Presence of abnormal crystals should be confirmed chemically before reporting.

(iii) Squamous and transitional epithelial cells if present in large numbers.

(iv) Large amounts of mucus.

(V) Any other abnormal finding.

Note - 0-5 leukocytes per hpf, 0-2 red cells per hpf, and 0-2 hyaline casts per Ipf are considered to be within normal limits.