A “dirty catch” urine sample is a specific and necessary collection method used for certain diagnostic tests, not a sign of poor hygiene. This sample involves collecting the first portion of the urine that is voided, often referred to as the first-void or first-catch urine. This technique is used when medical providers need to test for infections located primarily in the lower urinary tract, especially the urethra. This article will explain what this sample is, why this collection is necessary, and how to provide it correctly to ensure the most accurate test results.
Defining the Dirty Catch Method
The term “dirty catch” is a colloquial phrase used to distinguish this collection from the more common “clean catch” or “midstream” sample. The sample is considered “dirty” because its purpose is to intentionally capture the cellular debris and microorganisms that have accumulated in the urethra. This initial portion of urine effectively washes out the lining of the urethra, gathering any pathogens present there. Unlike other urine collections, there is no requirement to clean the genital area beforehand, as doing so could reduce the concentration of the target organisms needed for testing.
The dirty catch sample size is quite small, typically requiring only the initial 15 to 50 milliliters of urine passed. Collecting a much larger volume than this target range can dilute the sample, negatively impacting the sensitivity of the diagnostic test. This method is the opposite of a clean catch, where the patient discards the initial stream to avoid external contaminants. The first-void sample is deliberately concentrated with the contents of the urethra, making it a powerful diagnostic tool for localized infections.
Primary Diagnostic Applications
The reason for requiring a dirty catch sample is to detect infections that colonize the epithelial cells of the urethra, the tube that carries urine out of the body. The primary organisms targeted by this method are Chlamydia trachomatis and Neisseria gonorrhoeae. These organisms cause urethritis and their presence is highest within the first few drops of urine that flush the urethra.
The first-void urine provides the highest concentration of the organisms or their genetic material, which is necessary for highly sensitive testing. These samples are most often analyzed using nucleic acid amplification tests (NAATs), which detect the DNA or RNA of the target pathogens. For male patients, the first-catch urine is considered highly accurate, sometimes even superior to a urethral swab, because it is non-invasive. The technique may also be used to test for other infections causing urethritis, such as Trichomonas vaginalis, as the first-void sample offers excellent sensitivity for NAATs.
Step-by-Step Sample Collection Instructions
Proper collection of a dirty catch urine sample is paramount, and it begins well before the patient enters the collection room. To allow for maximum accumulation of the target organisms in the urethra, the patient must not urinate for a specific period before collection. Most healthcare providers recommend holding the urine for at least one to two hours prior to providing the sample. This ensures the first void is maximally concentrated with any organism-laden fluids.
When ready to provide the sample, the patient should open the sterile cup without touching the inside rim or the interior of the lid. It is important that the patient does not attempt to clean the genital area with any wipes or towelettes beforehand. The patient should then begin to urinate directly into the sterile collection cup, capturing only the very first portion of the urine stream.
The goal is to collect only the initial 15 to 50 milliliters, which is usually a small amount and may only partially fill the cup. Once this initial portion has been collected, the cup should be removed from the stream, and the patient should complete the rest of the urination into the toilet. Finally, the collection container must be securely capped, ensuring the lid is tightened completely, and then delivered promptly to the medical staff for processing. Delaying delivery may require the sample to be refrigerated, depending on the specific laboratory instructions.