Menstrual blood interfering with urine test results is a common concern. Although urine is sterile, the sample collection process can allow external fluids to mix with the specimen. Menstrual fluid contains blood, cells, and other matter that can alter the chemical and microscopic composition of a urine sample. This contamination can lead to inaccurate or invalid results, making interpretation difficult for healthcare providers. Understanding which tests are most susceptible to interference and how to minimize contamination is helpful for anyone needing a urine test during their cycle.
General Urinalysis and Physical Contamination
The most immediate impact of menstruation is on a standard urinalysis, which involves both dipstick and microscopic examination. The presence of menstrual blood causes a false positive result for hematuria, which is the medical term for blood in the urine. This interference occurs because menstrual blood contains the same components that would be abnormal if originating from the urinary tract itself, such as red blood cells.
Menstrual contamination can also significantly elevate protein levels in the sample, leading to a false indication of proteinuria. Elevated protein and blood readings can incorrectly suggest a urinary tract infection or kidney condition when none are present. The dipstick test, which relies on a chemical reaction, cannot distinguish between blood from the urinary system and blood from the vagina.
Laboratory technicians use microscopic analysis to help differentiate between contamination and true internal bleeding. The finding of a high number of squamous epithelial cells, which are cells shed from the external genital area, strongly suggests a contaminated sample. Conversely, true hematuria typically shows an elevated count of red blood cells per high-power field, often exceeding three to five cells, without excessive squamous cells. If the contamination is heavy, the sample may be deemed invalid, and the patient will be asked to repeat the test after their period has ended.
How Menstruation Affects Hormone and Drug Screening
Menstruation affects high-stakes testing, such as hormone and drug screening, primarily through sample dilution and chemical alteration. Urine pregnancy tests detect human chorionic gonadotropin (hCG), a hormone produced during pregnancy. The menstrual blood itself does not change the actual concentration of hCG in the body.
However, physical contamination with menstrual blood can dilute the urine sample, which may lead to a false negative result, especially in very early pregnancy when hCG levels are low. Blood can also chemically interfere with the antibodies used in the test strip, further compromising the accuracy of the reading.
Drug screening tests for metabolites of various substances are highly sensitive to specimen validity. These tests rely on specific gravity and creatinine levels to ensure the sample is not diluted or adulterated. Menstrual fluid contamination can alter the specific gravity of the sample, potentially flagging it as invalid. The slightly more alkaline pH of menstrual blood could also cause the urine sample to fall outside the normal pH range of 4.5 to 8.0, raising a suspicion of tampering.
A separate concern is the tendency for people to increase their fluid intake during menstruation, which may inadvertently dilute the sample. Dilution is identified by low creatinine levels and a specific gravity below the normal threshold of 1.005. If a drug screen comes back as “negative dilute,” it means the drug metabolites were present below the detection cutoff, often resulting in the requirement for a mandatory retest.
Ensuring Accurate Sample Collection
When a urine test cannot be rescheduled, using the clean-catch midstream technique is necessary to minimize contamination. The first step involves thoroughly cleansing the external genital area with the sterile wipes provided by the collection facility, wiping from front to back. This action removes surface contaminants, including menstrual fluid.
The next step is to insert a fresh tampon or menstrual cup just before collecting the sample; this helps block the flow of menstrual fluid into the collection cup. The person should begin urinating into the toilet first, then, without stopping the flow, place the collection cup into the stream to collect the midstream sample. Finally, the cup is removed, and the remaining urine is passed into the toilet.
If the menstrual flow is particularly heavy, or if the test is for a highly sensitive diagnostic purpose, such as kidney function monitoring, rescheduling the test may be the most reliable option. Always inform the healthcare provider that the sample was collected during menstruation, even if precautions were taken, to allow for the most accurate interpretation of the results.