From A to Z in Complete Urine Analysis (Urinalysis) (The method only)-Part Two
In this part we will talk about the microscopic examination of urine sediment and its significance
Why we should examine urine sediment microscopically?
Microscopic examination of urine sediments helps in the diagnosis of renal and urinary tract diseases with the chemical analysis by reagent strips (click here to see the chemical analysis of urine) where with microscopy
- One can detect those cellular and non-cellular elements of urine that do not give distinct chemical reactions.
- Microscopy can also serve as a confirmatory test in some circumstances (e.g., erythrocytes, leukocytes, bacteria).
- Useful for those samples with abnormal dipstick results.
For example if the reagent strip doesn’t give positive reaction for WBCs while the sample contain large amounts of WBCs, you will detect it during microscopic examination.
also if the reagent strip give positive result for WBCs, you can confirm this under microscope.
What items can we see in urine sediment under microscope?
The Chemist or clinician who is in training on urine analysis for first time will ask about what he search for in urine sediment under microscope and how he evaluate each item
The items you find in urine under microscope are classified as following:
A) Cellular Items:
Are derived from two sources:
(1) Spontaneously exfoliated epithelial lining cells of the kidney and lower urinary tract, and
(2) cells of hematogenous origin (leukocytes and erythrocytes).
B) Non-Cellular Items:
(2) Pollen grains
(3) Oil droplets
(4) Air bubbles
(6) Fecal contamination
What is the normal value for each item?
Actually there in no reference or normal values for items found in urine sediments because
(1) There is variation in the concentration of random urine samples in the same day where there is no normal reference value for each item in normal urine so it is better to use the first morning sample.
(2) there is no specific standard procedure for sediments concentration by centrifugation
Therefore individual laboratories should establish there own reference values …… that is good but how i can standardize the microscopic examination in my lab? its simple… there are some factors if you standardized them then you have a standard procedure for microscopic examination of urine sediments ….. they are according to National Committee for Clinical Laboratory Standards (NCCLS):-
- Volume of Urine examined: 12 ml is the recommended amount for examination (You should make your reference values using this volume using normal samples or commercial standards) …. if the sample volume is less than 12 ml, a corresponding factor must be applied to all numeric sediments counts and if the sample volume is more than 12 ml, mix it will then take 12 ml.
- Time of centrifugation: to ensure equal sedimentation of all specimens …. the recommended time for centrifugation is 5 minutes.
- Speed of centrifugation: the recommended speed is 400 g and according to the WHO the speed is 2000 g
- Volume of sediment examined: you should re-suspend the sediment in constant volume by distilled water.
- Reporting format: Every person at a given laboratory who performs a microscopic examination should use the same terminology, reporting format and reference ranges. Decisions about which formed elements should be reported and quantitated should be made by the individual laboratory based on the patient population and professional skill level of the people performing the testing.
Now I should ask you a question …. in case of the speed of the centrifugation, Is the speed of any centrifuge using the same rpm equal??? of course not because the speed depends on the Rotor Radius (from center of rotor to sample) in centimeters ….. That is good, but how you can calculate the speed of the centrifuge? you will use the following equation:-
- RCF = relative centrifugal force (expressed in in units of times gravity (× g))
- R = Rotor radius (i.e. the rotating disc of the centrifuge) in CM
- S =the speed of the centrifuge in rotations per minute (rpm)
For example, in the WHO manual, the speed for centrifugation to prepare the urine sediment is 2000 g (as we will see) and the rotor radius of your centrifuge is 7 CM, so to know the rpm you should adjust your centrifuge at, you can calculate it from the previous equation (2000 g = 1.118 x 10-5) 7 x S2), then the (S) = 5000 rpm.