Hemastix is a plastic reagent strip used in healthcare settings for the rapid, preliminary detection of blood in a urine sample. This test is a simple screening tool that provides immediate results, helping medical professionals quickly determine if further, more detailed analysis is necessary. The convenience of the dipstick format allows for bedside or in-office testing, making it a common first step in a complete urinalysis. The strip is designed to detect the presence of hemoglobin, the oxygen-carrying protein found in red blood cells, or myoglobin, a similar protein found in muscle tissue.
How the Strip Detects Blood
The detection of blood by the Hemastix strip relies on a specific chemical oxidation-reduction reaction catalyzed by the presence of hemoglobin. The reagent pad on the strip is impregnated with two main components: an organic peroxide, typically diisopropylbenzene dihydroperoxide, and a colorless dye called a chromogen, such as 3,3′,5,5′-tetramethylbenzidine (TMB). When the strip is dipped into a urine sample containing blood, the iron component within the hemoglobin acts as a powerful peroxidase enzyme. This peroxidase-like activity accelerates the breakdown of the organic peroxide, which then releases oxygen.
The liberated oxygen immediately oxidizes the colorless chromogen, causing a distinct color change. The reaction transforms the chromogen into a colored product, usually resulting in shades of green or blue on the test pad. The strip detects not only intact red blood cells (hematuria) but also free hemoglobin or myoglobin (hemoglobinuria or myoglobinuria).
Common Reasons for Testing
A healthcare provider will often order this test as part of a routine physical examination or when a patient presents with symptoms related to the urinary tract. The presence of blood in the urine, known as hematuria, can be a symptom of various conditions affecting the kidneys or the lower urinary system. The dipstick test is frequently used as a quick screen for suspected urinary tract infections (UTIs) or the presence of kidney stones, which can cause irritation and bleeding along the urinary tract. This test is also used to screen for more serious conditions, such as kidney disease, bladder tumors, or trauma to the urinary system. Because the test is a screening tool, a positive result does not provide a definitive diagnosis and always requires follow-up with a microscopic examination of the urine to confirm the presence of red blood cells and determine their quantity. Monitoring patients with chronic kidney conditions or those undergoing certain medical treatments is another common application for the test.
Interpreting the Color Scale
The degree of color change on the reagent pad corresponds directly to the concentration of blood detected in the urine sample. Results are interpreted visually by comparing the color on the strip to a standardized color chart printed on the bottle, which typically ranges from negative (yellow) to trace, small, moderate, or large amounts (green to blue). The deepest blue-green color suggests a high concentration of hemoglobin or red blood cells.
The physical appearance of the color change on the pad can offer an initial clue about the source of the blood. If the color change is uniform across the entire pad, it suggests the presence of free hemoglobin or myoglobin, which occurs when red blood cells have lysed, or broken open. Conversely, if the pad shows a speckled, or dotted, pattern of blue-green spots against a yellow background, it indicates that intact red blood cells are present. This speckled pattern is caused by the concentrated pockets of hemoglobin within the intact cells reacting with the reagents.
Causes of Inaccurate Readings
The chemical nature of the Hemastix test makes it susceptible to interference from other substances in the urine, leading to both false-positive and false-negative results. A false-positive result, where the strip indicates blood when none is present, can occur if the urine sample is contaminated with strong oxidizing agents, such as bleach or other cleaning solutions. Highly alkaline urine (pH of 9 or greater) can also induce a color change on the strip that mimics a true positive result.
False-negative results occur when blood is present but the strip fails to detect it. These are often caused by the presence of ascorbic acid (Vitamin C), which is a strong reducing agent that interferes with the oxidation of the chromogen, blocking the color-producing reaction. Highly concentrated urine, indicated by an elevated specific gravity, can also inhibit the reaction. Strips that have been improperly stored and exposed to light, heat, or moisture may lose their chemical reactivity over time.