Urine Culture Procedures and Analysis Guide

Feb 23, 2025

Urine Culture Procedures and Analysis

Overview

  • The video covers a series of four urine cultures.
  • These cultures include:
    1. Gram-negative urine
    2. Gram-positive urine
    3. Two mixed cultures with multiple colony types

Culture Setup

  • Urine is typically set up on two plates:
    • 5% Sheep Blood Plate
    • Bi-plate with McConkeys and CNA
  • Calibrated Loop Usage:
    • Delivers a fixed amount of urine (1/100th or 1/1000th mL)
    • Helps determine bacterial count (e.g., 10,000 vs. 100,000 colonies)

Reporting

  • Urine cultures are reported with specific numbers:
    • Ranges: <10,000, 10-20, 20-50, 50-100, >100,000 CFUs per mL
  • Terms used: Colony Forming Units (CFU) per mL

Media and Plate Types

  • CNA McConkeys Plate:
    • CNA: Selective for Gram-positive organisms and yeast; inhibits Gram-negative organisms
    • Differential based on hemolysis
  • McConkeys Plate:
    • Selective for Gram-negative organisms (Enterobacteriaceae, Pseudomonas)
    • Differential based on lactose fermentation (pink vs. clear colonies)

Example Culture Analysis

  • Gram-negative Urine:

    • No growth on CNA (indicating no Gram-positive organisms)
    • One colony type on McConkeys: pink, indicating lactose fermenters
    • Sheep blood plate used to count colonies (>100 colonies, reported as >100,000 CFUs/mL)
    • Likely organism: E. coli (common UTI cause)

  • Gram-positive Urine:

    • Growth on CNA, none on McConkeys
    • Greater than 100,000 colonies
    • Potential pathogens: Staph saprophyticus, Enterococcus, yeast (Candida albicans)
    • Observed features: "feet" around colonies, typical of yeast
    • Requires gram stain for confirmation

Collection Methods

  • Clean Catch / Midstream: Most prone to contamination
  • Indwelling Catheter / Suprapubic Aspirate: Lower contamination likelihood
  • Reporting considers contamination, colonization, or infection

Preliminary Reporting

  • For Gram-negative urine:

    • "Greater than 100,000 CFUs per mL of probable E. coli"
    • Identification and susceptibility testing to follow

  • For Gram-positive urine:

    • "Greater than 100,000 yeast identification to follow"

Key Takeaways

  • Understanding of media and plate types is crucial for identifying organisms
  • Importance of preliminary reports in clinical decision-making
  • Need for confirmation tests (e.g., gram stain) in ambiguous cases

These notes provide a comprehensive guide to the procedures and analysis discussed in the lecture on urine cultures.