If you have ever used a Hemastix strip, you have seen it change color when dipped in urine. The strip detects blood using a chemical reaction. Hemastix strips contain a substance called tetramethylbenzidine (TMB). When this substance touches hemoglobin — the protein in red blood cells — it reacts and changes color. The darker the color, the more blood is present. This guide explains exactly how that reaction works, what can cause false results, and what the test can and cannot tell you.
How Do Hemastix Strips Detect Blood in Urine at the Chemical Level?
The science behind Hemastix strips is straightforward. The test pad contains TMB and an organic peroxide. When hemoglobin from red blood cells touches the pad, it acts like a catalyst. It speeds up a reaction between the peroxide and the TMB. This reaction turns the pad blue or green.
The color change happens within 60 seconds. A light green or blue trace suggests a small amount of blood. A dark blue or green suggests a larger amount. The manufacturer includes a color chart on the bottle so you can compare the result.
Importantly, the test does not measure the number of red blood cells directly. It measures the peroxidase activity of hemoglobin. This means the test can detect both intact red blood cells and free hemoglobin. Free hemoglobin is hemoglobin that has leaked out of broken red blood cells. This distinction matters for understanding the cause.
What Does the Color Change Actually Mean?
The color scale on a Hemastix bottle runs from negative (no color change) to a deep blue-green. The result is not a precise measurement. It is a semi-quantitative estimate. A trace result is not the same as a small positive result.
The American Urological Association notes that trace results are common and often not clinically significant. A small positive result — 1+ or 2+ — requires follow-up. A large positive result — 3+ or 4+ — is a stronger signal that blood is present.
The test is designed to detect blood at levels as low as 0.015 to 0.062 milligrams of hemoglobin per deciliter of urine. That is a very small amount. A negative result is generally reliable. A positive result requires more testing to find the cause.
What Can Cause False Positives on Hemastix Strips?
False positives happen more often than people realize. The test reacts to hemoglobin, not just blood cells. Several things can trigger the reaction without actual bleeding in the urinary tract.
Menstrual blood is the most common cause of false positives in women. Even trace amounts of menstrual blood in the urine sample will trigger a positive result. The CDC recommends that women avoid collecting urine during menstruation unless the test is urgent.
Other causes include:
- Myoglobin. This protein is released from damaged muscle tissue. It is chemically similar to hemoglobin and triggers the same reaction. This can happen after intense exercise, a fall, or a muscle injury.
- Certain foods. Beets and rhubarb can sometimes cause a false positive. This is rare but possible.
- Oxidizing agents. Household bleach or hydrogen peroxide can contaminate a sample and cause a false positive. This is why clean collection is important.
- Dehydration. Concentrated urine can sometimes produce a trace result even when no blood is present.
False negatives are less common but can happen. Vitamin C (ascorbic acid) in high doses can block the reaction. This is a known limitation of the test. The CDC reports that vitamin C can cause false negative results at concentrations above 50 mg/dL of urine.
How Reliable Are Hemastix Strips Compared to a Urine Microscopy?
Hemastix strips are a screening test. They are not a diagnostic test. A urine microscopy — where a technician looks at the urine under a microscope — is the gold standard for confirming blood in urine.
Research published in the Journal of Urology found that Hemastix strips have a sensitivity of about 91 to 98 percent. That means they catch most cases of blood in urine. But their specificity is lower, around 70 to 85 percent. That means they produce a significant number of false positives.
Here is a comparison of the two methods:
| Feature | Hemastix Strip | Urine Microscopy |
|---|---|---|
| Time to result | 60 seconds | 30 minutes to 2 hours |
| Detects free hemoglobin | Yes | No |
| Detects intact red blood cells | Indirectly | Directly |
| False positive rate | 15 to 30 percent | Less than 5 percent |
| Cost | Low | Higher |
| Best for | Screening | Confirmation |
If you get a positive result on a Hemastix strip, your doctor will likely order a urine microscopy. This is standard practice. The microscopy can also show the shape of the red blood cells, which helps identify where the blood came from.
What Should You Do If You Get a Positive Hemastix Result?
A single positive result does not mean you have a serious condition. Many causes of blood in urine are harmless. But it does mean you need to follow up.
The National Kidney Foundation recommends the following steps after a positive Hemastix result:
- Repeat the test. Collect a fresh sample first thing in the morning. Avoid exercise for 24 hours before the test.
- Check for obvious causes. Have you had a urinary tract infection? Did you exercise intensely? Are you menstruating?
- See your doctor. A positive result on a repeat test requires a full evaluation. This usually includes a urine microscopy, a blood test, and sometimes imaging of the kidneys and bladder.
- Do not ignore it. Persistent blood in urine can be a sign of kidney stones, an enlarged prostate, a bladder infection, or — in rare cases — bladder or kidney cancer. Early detection improves outcomes.
It is worth noting that visible blood in urine — called gross hematuria — always requires immediate medical attention. Hemastix strips are not needed in that case. You can see the blood yourself.
Common Misconceptions About Hemastix Strips
There is a lot of misinformation about these strips. Here are the most common misunderstandings.
Misconception One: A positive result means you have a urinary tract infection. This is not true. Blood in urine can have many causes. An infection is just one of them. A positive strip alone does not diagnose a UTI. You need a urine culture for that.
Misconception Two: The darker the color, the more serious the problem. Not necessarily. The color intensity correlates with the amount of blood, not the severity of the cause. A kidney stone can cause a large amount of blood but is not life-threatening. A small amount of blood from a tumor can be more concerning. The amount of blood does not predict the diagnosis.
Misconception Three: Hemastix strips detect cancer. This is false. The strips detect blood. Blood in urine can be a sign of cancer, but it can also be a sign of many other things. The strips are a screening tool, not a cancer test. As of 2026, there is no clinical evidence that Hemastix strips can distinguish between cancerous and non-cancerous causes of blood.
Misconception Four: A negative result means your kidneys are healthy. This is also false. Hemastix strips only detect blood. They do not measure kidney function. You need a blood test for creatinine and eGFR to check kidney health. A negative Hemastix result does not rule out kidney disease.
Frequently Asked Questions
How do Hemastix strips detect blood in urine?
The strip contains tetramethylbenzidine and an organic peroxide. Hemoglobin from blood cells triggers a reaction that turns the pad blue or green.
Can Hemastix strips give false positive results?
Yes. Menstrual blood, myoglobin from muscle injury, certain foods, and household bleach can all cause false positives.
What should I do if my Hemastix test is positive?
Repeat the test with a fresh sample first thing in the morning. If it is still positive, see your doctor for a urine microscopy and further evaluation.
Are Hemastix strips accurate for detecting blood in urine?
They are accurate as a screening test but produce false positives 15 to 30 percent of the time. A urine microscopy is needed to confirm the result.
