|
 Introduction
Although easily adaptable manual methods are available
for urine analysis, not all intermediary-level laboratories have facilities
to prepare their own reagents. Sugar, albumin, urobilinogen
and bilirubin are the four biochemical substances
tested in a random urine sample. Although the heat and acetic acid test
detects the presence of proteins such as albumin, only a semiquantitative
test will be really useful. In the same way, Benedict's test, which is
commonly used, detects only the total reducing substance and does not predict
the amount of glucose present. The state-of-the-art technology is the use of
dipstick to detect biochemical substances in a convenient way. Many companies
are now manufacturing test strips based on the basic wet chemistry reactions
of the respective biochemical substances.
This section describes the routine screening tests done
using dipstick technology, highlighting the principal reactions and
limitations.
 Correct storage of strips
Protect the strips from moisture and excessive heat and
light but do not refrigerate. Replace the top on the storage container
immediately after removing a strip.
Glucose
Compared to Benedict's test, which detects the total
sugar present in urine, the strip test detects semi-quantitatively the amount
of glucose present in urine. It is a fast and convenient way of testing urine
to determine the amount of glucose present. Two types of dipstrips
are available, viz. Clinistix and Diastix .
These are fast and convenient ways of testing urine to determine the amount
of glucose excreted in urine.
Principle
 Clinistix
The strip isimpregnated with the enzymes glucose
oxidase and peroxides, and the indicator substance
O-toluidine. The 0-toluidine is oxidized to a
blue-green substance (Schiff’s base) with varying shades of colour, which is then compared with the standard chart
provided in the kitto report the approximate level of glucose present
in the urine.
Diastix
This strip has an area impregnated with the above
enzymes together with potassium iodide and a blue background dye. The oxygen
liberated in the final reaction binds with the dye to produce a series of colour changes 30 seconds after wetting the strip with
urine.
Procedure
Completely immerse the reagent area of the strip in
fresh urine for 1-2 seconds and remove. Gently tap the edge of the strip
against the side of the urine container to remove excess urine. Compare the
test area closely with a colour chart exactly 30
seconds after dipping the strip in the urine. Hold the strip close to the colour chart and match carefully.
Results
The results are expressed as either negative or varying
degrees of positive, indicating different amounts of glucose present.
Factors affecting sensitivity
Follow the instructions provided in the kit insert
carefully regarding the handling of strips so that the sensitivity of the
test will remain good. Exposure to atmospheric air reduces the sensitivity. pH and temperature are unimportant factors. Darkening of
the enzyme-coated area indicates loss of sensitivity. Hence discoloured strips should not be used.
False
positive
Contamination of glassware with oxidizng
agents such as sodium Hypochlorite and bleaching powder and detergents like
sodium phosphate will oxidize and change the colour
of chromogen in Clinistix.
False
negative
As ascorbic acid isan oxygen acceptor and most
likely to be present in large amounts in the urine of pregnant women, this
will cause a false-negative result.
These procedures will not detect the presence of other
reducing sugars, fructose, galactose, etc. in the
urine as they are specific for glucose.
Proteins
Several rapid screening tests are in routine use. The majority of the test stripshave been developed to
detect albumin and may be negative in the presence of other proteins, such as
Bence Jones Proteins.
Principle
It is based on the protein error of a pH indicator. At a
constant pH any colour change that happens to an
indicator is due to protein. The test area of the reagent strip is
impregnated with an indicator, tetrabromophenol
blue, buffered to pH 3.0. At this pH it is yellow in the absence of protein.
Protein forms a complex with the dye turning the colour
of the dye to green or bluish green.
Result
The colour iscompared
with the colour chart provided, which indicates the
approximate protein concentration.
A false-positive result may occur if:
the specimen is contaminated with vaginal or
urethral secretions
a strongly alkaline urine is used
the urine container is contaminated with
disinfectants such as chlorohexidine
False-negative results will be observed if acid has been
added to the urine as a preservative (for example in the estimation of
urinary calcium )
Multiple Reagent Strips
Test for glucose, bilirubin, ketones, specific gravity, blood, pH, protein and urobilinogen
Using a single stripan array of
semi-quantitative tests listed above can be done.
Principle
It is a firm plastic strip to which are affixed several
separate reagent areas. Depending on the reagents being used, these strips are
employed for tests indicated above.
Glucose: It makes use of the same
principle as described for Diastix, the final colour ranging from green to brown.
Bilirubin:
It is based on the coupling of bilirubin with
diazotized dichloronaniline in a strongly acid
medium. The colour ranges through various shades of
tan.
Ketone:
It is based on Rothera's reaction principle and on
the development of colours, ranging from buff-pink
for a negative reading to purple when acetoacetate
reacts with nitropruside. It also detects acetone
but not beta-hydroxybutyrate.
Specific gravity :
In the presence of an indicator the polyelectrolytes
present in urine give colours ranging from deep
blue green in urine of low ionic concentration through green to yellow green
in urine of increasing ionic concentration.
pH:
This test is based on the double indicator principle that gives a broad range
of colours covering the entire urinary pH range. Colours range from orange through yellow and green to
blue.
Protein: It is based on the protein
error of the pH indicator. At a constant pH, the presence of protein leads to
the development of any green colour. Colours range from yellow for "negative"
through yellow green and green to green blue for "positive"
reactions.
Uroblilinogen:
This test is based on a modified Ehrlich reaction, in which p- dimethyl amino benzaldehyde in
conjunction with a colour enhancer reacts with urobilinogen in a strongly acid medium to produce a
pink-red colour.
Procedure
Do not use discoloured strips.
Do not touch the test areas. Dip the test areas of the strip in urine
completely, but briefly, to avoid dissolving out the reagents. Read the test
results carefully at the time specified in good light and with the test area
held near the specimen appropriate colour chart on
the bottle label. Do not read the strips in direct sunlight.
Interference
|
Glucose:
|
Ascorbate and Ketones may cause
a false-negative result.
|
|
Bilirubin:
|
Indican (ureloxyl sulfate ) will cause a false-positive result, while ascorbate will cause a false-negative result.
|
|
Ketone:
|
Pigmented urine or urine containing levodopa
metabolites/sulph-hydroxyl drugs may cause a
false-positive result.
|
|
Protein:
|
Refer to page 105 under Dipstick - Protein, (b) Result
|
|
Quality Control:
|
Dipstix for glucose and proteins are generally
recommended for intermediate as well as peripheral laboratories for routine
urine screening. However, it is good to cross-check occasionally the
performance of the strips by comparing with conventional methods described
under Section 5.3.Urinalysis – Semiqualitative
tests. It is also good to check the strip results with the patient’s other
relevant biochemical results.
|
|
