Optimal urine collection for culture should be a mid stream specimen.
In an ideal world – a mid stream specimen is obtained into a sterile receptacle, with foreskin retracted or labia separated.
Vaginal epithelial cells or lactobacilli on analysis are diagnostic of contamination. High squamous epithelial cells also suggest potential contamination.
Factors which may preclude optimal collection:
- Increased BMI
- Post menopausal atrophy
- Unable to stand / weight bear
- Intravaginal pessary
- Non sterile collection container
- Poor dexterity
- Phimosis
Specimens should not be taken from indwelling catheters or stomas – an ideal specimen is obtained from a fresh catheter or from catheterising the stoma.
Urine dipstick analysis
Leukocyte esterase is produced by the breakdown of white blood cells in the urine – therefore its presence is an indication of pyuria, but not necessarily bacteriuria
Presence of white cells in urine is common in catheterisation and other inflammation (even appendicitis etc)
Nitrites are present when bacteria reduce dietary nitrates via nitrate reductase
Not all bacteria produce nitrites – in particular, gram positive, pseudomonas and Acinetobacter
Patients with low nitrate in their diet may also have false negative
Positive nitrites are the most helpful independent variable on a dipstick in conjunction with UTI symptoms.
A negative urinalysis has a pretty high NPV for UTI, especially in the absence of symptoms.
Downsides of dipsticks:
- Nitrites and other reagents are sensitive to air – false positive if lids left off
- Leukocyte esterase reaction takes up to 5 minutes
- Relying on urinalysis alone will overdiagnose UTIs
Blood often false positive (menstruation, myoglobin)