Urinalysis - The Carter Center

Since all these tests are based on the peroxidase activity of hemoglobin,
other substances with peroxidase activity also give positive reactions in
the tests.
Factors that Affect Hemoglobin Determination
False negative
- High specific gravity such as heavy proteinuria (over 5 g/1). This
prevent lysis of RBCs and may reduce the color reaction.
- Low to false negative results are obtained if the urine contains
large amounts of ascorbic acid.
- Nitrite delays test reaction.
- Formaline used as preservative, fix the cell and prevent hemolysis.
False positive
- Low specific gravity < 1.010 enhances lysis and produces color
reaction.
- Microbial peroxidase produces false the color reaction.
- False positive reactions can result from the presence of
contaminating oxidizing detergents on the urine such as bleach.
A. Guaiac Test
Procedure:
1. Pour into a test tube 4 drops of urine and a few drops of
concentrated acetic acid.
2. Pour the following into a second test tube
- A knife point of Guaiac
- 2 ml ethanol (95%)
- 2 ml fresh 3% H 2 O 2
Mix the above slowly and pour the same into the side of the urine
tube.
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