Introduction
A urinary tract infection (UTI) is an infection in any part of the urinary system, including the kidneys, ureters, bladder or urethra. UTIs are one of the most common bacterial infections, affecting around 150 million people worldwide each year. Women are more prone to UTIs than men due to their shorter urethras. The most common symptoms of a UTI include a strong urge to urinate frequently, a burning sensation when urinating, cloudy or foul-smelling urine, and pelvic pain or tenderness.
To diagnose a UTI, doctors often use a simple urine dipstick test. This involves dipping a thin plastic strip with various chemical pads into a urine sample. The test strip then changes color to indicate the presence of certain substances that may signify an infection. Some of the key things that can show up on a urine strip if you have a UTI include:
Leukocyte esterase
One of the most telling signs of a UTI on a urine dipstick is a positive result for leukocyte esterase. Leukocyte esterase is an enzyme that is released from white blood cells (leukocytes) when they are actively fighting an infection. The presence of leukocyte esterase in urine indicates that white blood cells have traveled to the urinary tract to battle invading bacteria.
A urine dipstick contains a leukocyte esterase pad that will change color in the presence of the enzyme. A positive leukocyte esterase test is highly indicative of a UTI, with a sensitivity of over 90%. The more leukocyte esterase detected, the more white blood cells there are, signaling a more serious bacterial infection. False positives can occasionally occur if vaginal discharge gets into the urine sample.
Nitrites
Another key indicator of a UTI on a urine test strip is a positive nitrite result. Many common UTI-causing bacteria like E. coli are able to convert nitrates in the urine to nitrites. Normally urine contains nitrates but little or no nitrites.
The urine dipstick has a nitrite pad that will change color if this conversion process has occurred, signalling bacterial growth. A positive nitrite test has a specificity of over 95% for diagnosing a UTI when symptoms are present. However, the sensitivity can be quite low, around 45-60%, so a negative nitrite result does not rule out a UTI. This is because some UTIs are caused by bacteria that do not form nitrites. The test may also be negative if the infection is very recent.
Protein
Protein in the urine, known as proteinuria, can be another possible sign of a UTI on a dipstick test. Normally only tiny traces of protein are excreted into urine. Higher levels can indicate kidney inflammation or damage due to a severe kidney infection (pyelonephritis).
The urine strip contains a protein pad that will change color in the presence of excess protein. The amount of color change correlates with the quantity of protein. 1+ or 2+ protein may be seen with a lower tract UTI involving the bladder and urethra. However, higher levels of 3+ or 4+ often indicate upper tract involvement and kidney damage. Proteinuria can also occur with other conditions like diabetes, so it is not a definitive indicator of a UTI on its own.
Blood
Hematuria, or the presence of red blood cells in the urine, can occasionally occur with a UTI. Normally urine has no visible blood. The dipstick test strip has a blood pad that will change color if excessive red blood cells are detected.
The inflammation and irritation of the bladder and urethral lining caused by an infection can sometimes cause bleeding into the urine. However, hematuria is an uncommon finding in simple lower tract UTIs. It occurs more frequently with acute kidney infections. The blood may be visible to the naked eye in more severe cases. As with proteinuria, hematuria can also result from other medical problems like kidney stones or autoimmune conditions.
Ketones
In some cases, small or moderate amounts of ketones may show up in the urine during a UTI. Ketones are produced when the body breaks down fat for fuel during prolonged fasting, restricted diets, excessive vomiting, or uncontrolled diabetes. The dipstick test contains a ketone pad that changes color in response to increased ketone bodies.
Ketones may occur with a UTI due to increased metabolism, decreased food intake, and vomiting in severe infection. Pregnancy, diabetes, and eating disorders can increase susceptibility to ketonuria during infections. However, significant ketonuria is uncommon with routine UTIs. High levels often correlate with dehydration and electrolyte disturbances.
Glucose
Glucosuria, or glucose spilling over into the urine, may be seen in very severe cases of UTI with kidney involvement. Normally little or no glucose is present in urine. The glucose pad on a urine dipstick will change color at levels above the renal threshold.
In kidney infections, the renal tubules become damaged and unable to reabsorb glucose properly, leading to glucose overflow into the urine. Marked glucosuria usually correlates with diabetic ketoacidosis but can also signal acute kidney injury from pyelonephritis. Mild glucosuria can occasionally result from pregnancy, certain medications, or renal tubular disorders.
Urobilinogen
A urine dipstick may also check for urobilinogen levels. Urobilinogen is a byproduct of the breakdown of aged red blood cells. Small amounts are normally excreted in urine and stool. Some dipsticks for UTIs include a urobilinogen pad to check for liver conditions like hepatitis and cirrhosis.
With upper urinary tract infections that cause inflammation of the kidneys and liver, mild increases in urobilinogen can sometimes occur. This is likely due to accelerated red blood cell turnover from the infection. Markedly elevated levels need further evaluation for liver disease. Normal results do not rule out a UTI.
pH
The urine pH may also be assessed with a dipstick test. Normal urine pH ranges between 4.5-8, with an average of around 6. Changes in urine pH can indicate metabolic disturbances in severe UTIs.
Acute kidney infections, urosepsis, and increased acid production from bacteria can sometimes lower urine pH. Very low pH below 5.5 may indicate metabolic acidosis during a severe kidney infection. High pH above 7.5 can also occur from certain bacteria that split urea and release ammonia. However, urine pH has low sensitivity for UTIs and is an indirect finding.
Specific gravity
Refractometric urine dipsticks measure urine specific gravity, or the concentration of dissolved particles. Specific gravity typically ranges from 1.003-1.03. In UTIs, the kidneys’ ability to concentrate urine may become impaired.
As a result, chronic or severe kidney infections can cause dilute urine with low specific gravity, below 1.010. This corresponds to inadequate renal concentration. However, non-specific causes like excess fluid intake can also lead to dilute urine. An elevated specific gravity may be seen with dehydration or kidney damage.
Summary of Key Urine Dipstick Findings in UTI
| Test | Normal Range | UTI Finding | Indicates |
|---|---|---|---|
| Leukocyte Esterase | Negative | Positive | White blood cells in urine |
| Nitrites | Negative | Positive | Bacterial conversion of nitrates |
| Protein | Negative or trace | 1+ or greater | Kidney inflammation |
| Blood | Negative | Positive | Bladder/urethral bleeding |
| Ketones | Negative | Positive | Increased metabolism |
| Glucose | Negative | Positive | Kidney damage |
| Urobilinogen | Normal range | Possibly increased | Liver impairment (rare) |
| pH | 4.5-8 | Below 5.5 | Acidosis |
| Specific Gravity | 1.003-1.030 | Below 1.010 | Kidney impairment |
Causes of abnormal urine findings besides UTI
While the urine strip results described above can often indicate a likely UTI, it is important to note that non-infectious conditions can also cause similar abnormalities in urine tests.
For example:
- Proteinuria can result from kidney disorders, high blood pressure, diabetes, or heavy exercise.
- Hematuria may be caused by kidney stones, enlarged prostate, endometriosis, trauma, or cancer.
- Glucosuria can occur in diabetes, pregnancy, or with certain medications.
- Ketones may appear due to fasting, vomiting, low carbohydrate diet, or alcoholism.
- pH changes can result from diet, medications, renal tubular acidosis, or kidney failure.
Therefore, the entire clinical presentation including symptoms should be considered, not just the urine dipstick findings. Follow-up microscopic urinalysis and urine culture may be needed for confirmation. Nonetheless, the urine strip test remains a rapid, inexpensive, and helpful initial laboratory screen for UTIs when interpreted in context.
When to seek medical treatment based on urine dipstick results
If you are experiencing UTI symptoms like burning urination, pelvic pain or urine frequency, and your urine strip shows significant findings like:
- Positive leukocyte esterase
- Positive nitrites
- Hematuria
- 3+ protein or glucose
You should contact your doctor promptly for further evaluation and treatment with antibiotics. Positive leukocyte esterase and nitrites with symptoms indicate a likely acute UTI needing antibiotics. Hematuria, high protein, glucose or ketones may signal a kidney infection which can progress rapidly to serious disease. Quick medical attention is advised.
Milder or equivocal results like trace protein or ketones may just require increased hydration and retesting. Negative strips do not rule out early UTI, as bacteria may not yet show up. With recurrent UTI symptoms and negative dipsticks, a microscope urine culture may be warranted to check for bacterial growth. Follow your doctor’s advice on whether additional testing or antibiotic treatment is needed based on your urine findings in context of your symptoms.
Conclusion
Urine dipstick testing provides a fast and easy way to screen for possible signs of a urinary tract infection. Key findings that can indicate a UTI include positive leukocyte esterase, positive nitrites, protein, blood, high ketones and glucose. However, non-infectious conditions can also cause similar urine abnormalities. The entire clinical picture and combination of UTI symptoms should be considered, not just the urine strip results. Speak to your doctor about appropriate antibiotic treatment or further testing if your urine dipstick results and symptoms suggest a possible active urinary tract infection requiring medical care.
