First catch urine refers to the initial portion of the urinary stream collected for diagnostic testing. This collection method is distinct from the more familiar “midstream clean catch” often requested for general urinary tract infections. The first catch technique is a specialized method used when the test aims to detect microorganisms or biomarkers concentrated in the lower part of the urinary tract, such as the urethra. This ensures the sample contains the highest concentration of the target material for an accurate analysis.
Defining First Catch and Its Diagnostic Rationale
The primary reason for collecting the first voided urine is to maximize the concentration of diagnostic material originating from the urethra and lower genital tract. Unlike a midstream sample, where the initial flow is discarded to reduce external contamination, the first catch sample is intentionally collected to “wash out” cells and microorganisms residing in the lowest part of the tract. This initial stream flushes out infectious organisms that have accumulated in the urethra since the last time the patient urinated.
This mechanism is effective for diagnosing infections that primarily affect the urethra. If a midstream sample were collected, the infectious material would be diluted by cleaner urine from the bladder, significantly decreasing the probability of detection. First-catch urine contains a substantially higher organism load compared to midstream samples, which improves the sensitivity of molecular testing.
Step-by-Step Collection Procedure
Correct collection of a first catch urine sample requires specific preparation. Patients are typically instructed not to urinate for a specified time before the test, with many protocols recommending a minimum of one to two hours without voiding. This retention time allows for the accumulation of cells and organisms within the urethra.
When ready to collect the sample, the patient should not use any cleansing wipes or attempt a “clean catch” technique. Cleansing the genital area can remove the very organisms the test is trying to detect, potentially leading to a false negative result. The patient should begin the urinary stream and immediately collect only the initial portion directly into the sterile specimen cup.
The volume collected should be small and specific, generally between 10 and 50 milliliters. Collecting more than the required amount can dilute the sample and reduce the test’s accuracy. After collecting the initial stream, the patient must finish voiding the rest of the urine into the toilet. The sealed cup should be sealed tightly and delivered for testing promptly.
Common Diagnostic Uses
The first catch urine sample is routinely used for Nucleic Acid Amplification Tests (NAATs) to detect the genetic material of infectious organisms. The most frequent application for this sample type is the screening and diagnosis of urogenital infections. This collection method is the preferred non-invasive option for testing in men.
Specifically, first catch urine is widely accepted for the detection of Chlamydia trachomatis and Neisseria gonorrhoeae, two of the most common sexually transmitted infections. These organisms cause infections that reside in the urethra, making the first voided urine an ideal specimen for their detection. While first catch urine is acceptable for screening in women, self-collected vaginal swabs often offer slightly higher sensitivity for these infections. The utility of this collection method extends beyond these common infections, as it is also being explored for its ability to detect other biomarkers and pathogens from the lower urinary tract.