8.2 Tests for Urinary Proteins
8.2.1 Dipstick Test
This is the most common test for proteinuria. Dipsticks are plastic strips impregnated
with tetrabromophenol blue buffered to pH 3.0 with citrate. The test strips remain
yellow in the absence of protein but changes to blue through various intermediate
shades of green in presence of increasing protein concentration. This is due to
divalent anionic form of indicator dye combining with proteins causing further
dissociation of yellow monovalent anion into blue divalent anion. The blue-green
color produced is directly proportional to the concentration of proteins in the
specimen.
8.2.1.1 Method
Dip the strip in the urine for a second and remove the excess urine by tapping the
edge against the container. Compare the color with the test chart within 30–60 s. The
dipstick tests are more sensitive to albumin while Bence-Jones proteins, globulins,
and glycoproteins are less readily detected. Dipstick is useful if urinary proteins are
300–500 mg/day or albumin >10–20 mg/dl.
8.2.2 The Boiling Test for Coagulable Proteins
When proteins are heated at pH equal to isoelectric point (pI), they get coagulated
and denatured.
8.2.2.1 Reagents
33% (v/v) aqueous solution of acetic acid.
8.2.2.2 Procedure
Make the urine slightly acidic with acetic acid if urine is alkaline. Fill a test tube
(3/4th) with acidified urine and heat the upper (1/3rd) portion until it boils. Compare
it with unboiled urine lower down the tube. Appearance of faint turbidity to heavy
precipitate indicates positive test. Phosphates also give precipitates, so to rule out
false positive result, acetic acid is added. Phosphate get dissolved leaving any
coagulated protein still visible.
Note Boiling test is semiquantitative test, so it can be used to measure concentration
of proteins to some extent. No turbidity indicates negative result; only turbidity
indicates˂30 mg/dl; turbidity with precipitates indicates 100 mg/dl, while turbidity
with coagulation indicates proteins >300 mg/dl. If heavy coagulate appears, that
indicates >2 g/dl proteins in urine sample.
34 8 To Perform Qualitative Tests for Urinary Proteins