Urinalysis refers to a series of tests that can be run to assess the chemical quality of urine. Typically, this is done by dipping a strip into the urine sample, and chemical reactions change the color of the pads.
The dipstick test for blood is based on the peroxidase-like activity of hemoglobin. Red cells are lysed on contact with the strip, allowing free hemoglobin to catalyze the liberation of oxygen from organic peroxide. Tetramethylbenzidine is oxidized, producing a color change from orange to green-blue. If intact red cells do not lyse, they may produce speckles on the pad.
A few notes on reference ranges:
- The reference range for RBCs in normal urine is 0-3 RBC/hpf in males and 0-12 RBCs/hpf in females when concentrated urine sediment is examined. This range corresponds to a concentration of 3 to 20 RBCs/uL of urine. Dipstick sensitivity extends into the reference range. Therefore, trace to 1+ reading may be obtained on urine from as many as 3% of healthy individuals.
- The sensitivity of dipsticks for hemoglobin is 0.015 to 0.062 mg/dL. This concentration corresponds to 5 to 21 RBCs/uL or 1 to 4 RBCs/hpf of concentrated urine sediment.
Occult blood is blood that cannot be seen without a microscope. Normal urine doesn’t contain red blood cells, except females’ urine during menstruation. Blood in the urine, called hematuria, is not a normal finding, but it is not uncommon and not necessarily a cause for alarm. Your healthcare professional will investigate further to try to determine the source and underlying cause of the blood.
There are some risk factors that can put you at risk for hematuria such as:
- Age (men older than 50)
- Sex (women with urinary tract infections, men with kidney stones)
- Recent infection (particularly of the kidneys in children)
- Family history
- Certain medications (aspirin or penicillin)
- Strenuous exercise
A low, or negative, urine occult blood test is normal and healthy, as it indicates that there is no blood in the urine.
False Negative Dipstick:
- Dipsticks exposed to air
- RBCs settle out & urine not mixed
- Ascorbic acid (high concentration)
- Formaldelhyde (preservative tablets)
- High specific gravity
- Very high protein
- Urine pH <5.1
- High nitrite from UTI
- Captopril (Capoten)
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In healthy individuals, fewer than 1000 red cells are excreted in the urine per minute. When 3000 to 4000 red cells are excreted per minute, 2 to 3 red cells will be seen per high power field, indicating microscopic hematuria. Gross hematuria occurs when more than 1 million red cells are excreted per minute. Hematuria can be due to lesions within the GU tract involving the kidneys, ureters, bladder, prostate, or urethra.
The most common disorders include cancer, kidney stones, renal disease, urinary tract infection, and benign prostatic hyperplasia. Transient hematuria can result from menstruation, viral illnesses, strenuous exercise, and mild trauma. Anticoagulant therapy and chemotherapy may also cause hematuria. No etiology can be determined in approximately 45% of cases of microscopic hematuria.
A positive dipstick test for blood does not tell whether the reaction is due to red cells, red cell casts, hemoglobin casts, or myoglobin. Many conditions can lead to discrepant dipstick and microscopic findings. Any situation that causes red cell hemolysis will give a positive dipstick and negative microscopic result. Urine should be tested shortly after collection because red cell lysis may occur as the sample ages, if the pH is alkaline, or if the specific gravity is 1.010 or less. Bacterially contaminated urine specimens may contain sufficient peroxidase activity to produce a false positive reaction. False positive reactions can also be caused by vegetable peroxidase.
False Positive Dipstick:
- Oxidizing agents - bleach, detergent, iodine
- Bacterial peroxidase
- Vegetable peroxidase
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