A positive result for occult blood in a routine urinalysis signifies the presence of blood not visible to the naked eye. The word “occult” means hidden, referring to blood detected only through laboratory testing. This microscopic presence of blood is medically known as microscopic hematuria, which prompts an investigation into its source. For people who menstruate, a frequent, non-disease-related explanation is external contamination of the urine sample. Understanding the distinction between a true medical issue and a collection error is the first step in addressing this common clinical situation.
What Occult Blood in Urine Means
Occult blood in urine signifies that red blood cells are present in the sample at a level too low to change the urine’s color. The most common initial test is a urine dipstick, which uses a chemical reaction to detect hemoglobin.
A positive dipstick result prompts a microscopic analysis of the urine, which is the definitive test to confirm the presence of red blood cells. Technicians examine the sample under a high-power field (HPF) to count the cells. Finding three or more red blood cells per HPF is considered significant microscopic hematuria, warranting further medical attention. A positive test indicates blood components are present but does not specify if the source is the urinary tract or external contamination.
How Menstruation Affects Urine Test Results
Menstrual blood contamination is the most common cause of a false positive occult blood result in people who menstruate. Because the urethra is close to the vaginal opening, menstrual blood easily mixes with the urine stream during collection. This external blood introduces red blood cells and hemoglobin, leading to a positive reading that incorrectly suggests a problem within the urinary tract.
Contamination can also mimic true hematuria or suggest a urinary tract infection (UTI). Menstrual fluid contains white blood cells, which can trigger a positive result for leukocyte esterase on the dipstick. Healthcare providers routinely ask about the timing of the last menstrual period when interpreting results, and if contamination is suspected, the positive result is typically considered a false positive.
Other Reasons for Occult Blood
While external contamination is frequent, occult blood can also signal an issue within the urinary system.
Common Causes
One of the most common causes is a urinary tract infection (UTI), where bacteria inflame the urinary tract lining, leading to microscopic bleeding. Kidney or bladder stones are another frequent cause, as these mineral deposits scrape the lining of the urinary tract as they pass. Even strenuous physical activity, such as long-distance running, can temporarily cause benign microscopic hematuria that resolves on its own.
Serious Conditions
Occult blood can also be a sign of more serious conditions. These include kidney diseases, such as glomerulonephritis, or cancers of the bladder or kidney. Systemic causes of microscopic bleeding, though rare, include sickle cell disease or bleeding disorders.
Following Up on a Positive Test Result
If a positive result for occult blood is found, the first step is to communicate with the healthcare provider about the possibility of menstrual contamination. If contamination is suspected, the most common action is to simply repeat the urinalysis. The retest should be scheduled for a time when the individual is not menstruating, preferably at least 48 hours after the bleeding has completely stopped.
To minimize the chance of contamination during the retest, a “clean-catch, midstream” collection technique is necessary. This involves thoroughly cleaning the genital area with provided wipes, starting to urinate into the toilet, and then catching the middle portion of the urine stream in the sterile collection cup. If the second test still shows occult blood, or if the individual is experiencing other symptoms like pain during urination, further diagnostic tests are necessary. These may include imaging tests, such as an ultrasound or CT scan, or a procedure called a cystoscopy to visually inspect the bladder’s interior.