A urine dipstick test is a rapid screening method used to detect signs of a urinary tract infection (UTI) or other conditions affecting the urinary system. The tool uses small chemical pads attached to a strip that react to specific compounds in the urine, causing them to change color. This color change provides preliminary information about the urine sample’s chemical composition. This quick assessment helps guide the need for more definitive laboratory testing, such as a formal urine culture.
Identifying Bacterial Presence: Nitrites and Leukocyte Esterase
The two most direct indicators of a possible UTI on a dipstick are positive results for nitrites and leukocyte esterase. A positive result for both markers is highly suggestive of a bacterial infection within the urinary tract. The pads on the strip change color—often to pink or purple for nitrites and purple for leukocyte esterase—to signal the presence of these compounds in the sample.
Nitrites serve as a strong chemical indicator of bacterial presence. Certain common UTI-causing bacteria, particularly gram-negative organisms like Escherichia coli, possess an enzyme called nitrate reductase. This enzyme converts nitrates, which are naturally present in the urine, into nitrites. The conversion typically requires the urine to remain in the bladder for at least four hours, which is why first-morning samples are often preferred for testing.
A positive nitrite result is highly specific for bacteriuria, meaning bacteria are almost certainly present when detected. However, a negative nitrite result does not rule out an infection, as some UTI bacteria, such as Enterococcus species, do not produce the necessary enzyme to convert nitrate into nitrite. A positive nitrite result provides immediate evidence of an active infection caused by nitrate-reducing bacteria.
Leukocyte esterase (LE) is the second primary marker, indicating the body’s inflammatory response. LE is an enzyme released by activated white blood cells (leukocytes), specifically neutrophils, which are sent to the site of infection. The presence of LE in the urine suggests pyuria, an elevated level of white blood cells in the urinary tract.
A positive LE result signals that the immune system is actively fighting an invader, commonly a bacterial UTI. Unlike nitrites, LE can be positive due to other forms of inflammation, such as kidney stones or non-bacterial infections. The combination of positive nitrite and leukocyte esterase tests offers the highest predictive value for diagnosing an active bacterial UTI.
Secondary Indicators: Blood and Protein
While nitrites and leukocyte esterase are the direct infection markers, the dipstick also tests for secondary indicators like blood and protein, which often accompany a UTI. The presence of blood, or hematuria, is a common finding in UTIs, caused by irritation and inflammation of the urinary tract lining. This irritation leads to the microscopic presence of red blood cells, which are detected by the dipstick pad.
The dipstick pad for blood detects both intact red blood cells and free hemoglobin, making it a sensitive screening tool. Blood in the urine can also be caused by kidney stones, kidney disease, or strenuous exercise. Therefore, it is considered supportive evidence rather than a definitive sign of infection. A positive blood result without positive nitrites or LE suggests a cause other than an acute bacterial infection.
Protein in the urine, or proteinuria, is another marker that can be temporarily elevated during a severe UTI. While high levels of protein usually signal underlying kidney disease, mild proteinuria can occur due to the inflammatory process associated with the infection.
Like blood, a positive protein result is not specific to a UTI and can be caused by various medical conditions. When found alongside positive nitrites and leukocyte esterase, a mild elevation in protein adds weight to the suspicion of an acute infection. Without the primary infection markers, a positive protein result directs investigation toward chronic kidney conditions.
When the Dipstick Misleads: False Results and Limitations
The urine dipstick is a screening tool, and its results are not always accurate, leading to potential false positives and false negatives. A false negative occurs when an infection is present but the dipstick test is negative. This can happen if the urine is overly diluted, lowering the concentration of nitrites and white blood cells below the strip’s detection limit. Additionally, ascorbic acid (Vitamin C) can chemically interfere with the reaction pads, leading to false negative results for both nitrites and leukocyte esterase.
False negatives also result if the infection is caused by bacteria that do not convert nitrate to nitrite, such as Enterococcus, or if the urine has not been held in the bladder long enough for conversion. Conversely, a false positive result indicates an infection when none is present. This is often caused by improper sample collection, where contamination from external skin bacteria or vaginal secretions enters the sample.
The dipstick test is not designed to replace comprehensive laboratory analysis. Even with strongly positive results, definitive diagnosis and the choice of appropriate antibiotic treatment rely on a formal urine culture and microscopic analysis. The culture identifies the specific organism causing the infection, confirming the dipstick’s findings and ensuring effective medical management.