Urine Trouble

Confusion

We use a lot of terms in referring to urine sample collection: first-catch, clean-catch, midstream, etc. It can be confusing, but most terms simply refer to the tests we anticipate sending (e.g., chlamydia/gonorrhea NAAT, urine culture). Let’s review: 

First-catch urine

A first-catch urine (preferred term in CDC’s 2014 MMR on chlamydia/gonorrhea testing) is also known as a first-void or first-pass urine, or even dirty urine (this term is not preferred). It typically refers to the first 15-20 mL of urine voided (for context, our urine containers hold up to 100 mL). In the presence of chlamydia or gonorrhea infections, first-catch urine samples of this volume have been shown to contain higher concentrations of bacterial DNA. A first-catch urine may be collected at any time of day (in contrast to a first-morning urine specimen) so long as the patient has not urinated for at least 20-60 minutes (most studies use 60 minutes as the minimum time). First-catch samples do not require skin cleansing prior to collection because cleansing may (slightly) reduce testing sensitivity.  

Other notes:

  • Urine vs urethral swabs: “First catch urine samples are equivalent to, and in some situations superior to, urethral swabs. Use of urine samples is highly acceptable and might improve the likelihood of uptake of routine screening in men”
  • Urine vs vaginal/cervical swabs: “First catch urine… might detect up to 10% fewer infections when compared with vaginal and endocervical swab samples”
  • Trichomoniasis: First-catch urine samples appear to have excellent sensitivity and specificity on NAATs, similar to vaginal and urethral swabs

Clean-catch midstream urine

A clean-catch urine is the most commonly collected sample for urine culture testing. A midstream sample is recommended, theoretically allowing the initial stream of urine to clear urethral contaminants. Nevertheless, there is no clear evidence that a clean-catch midstream urine yields more accurate results than a midstream catch without prior cleansing or random urine sampling collected without any prior instruction. Interestingly, the likelihood of detecting bacteriuria from a voided sample is actually highest in a first-morning urine sample, which is more likely to be concentrated (i.e., bacteria in the bladder will have had time to multiply overnight), but is often less practical to obtain in a clinic setting. 

Collecting a clean-catch midstream sample requires a few steps, which patients should be instructed on in advance: 

  • Cleansing:
    • After washing their hands with soap and water, patients should then disinfect the area around the urethral meatus with an antiseptic solution. In clinic, we provide towelettes that contain castile soap. Standard practice is to use 3 towelettes, one after the other but allowing the area to dry between each application. Instructions on cleansing depends on a patient’s anatomy:
      • Vagina/vulva: Patients should spread the labia and wipe front to back with each towelette. 
      • Penis: Patients should clean the entire glans. Patients who are uncircumcised should retract their foreskin prior to cleansing (and maintain foreskin retraction while urinating). 
  • Urinating:
    • After cleansing, patients should start urinating into the toilet and then collect a midstream specimen in the specimen container, taking care to not touch the inside of the container. 
    • There is no standard volume for collection, although many laboratories suggest 30-50 mL. 
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