Module 4- urinary tract infections – annotated SAQ

Urine bench Ferguson 2013: summary to go with your reading of the relevant text chapter.


 1.       Discuss sterile pyuria

Presence of white cells (>10/10^9/L) without growth of conventional uropathogens.     Causes include:

  • UTI- patient may have been on antibiotics and the urine has been rendered sterile
  • Interstitial nephritis – phase contrast microscopy does not distinguish neutrophils from tubular epitelial cells.
  • Fastidious organism present that requires special growth requirements or duration – classically, this is genito-urinary tuberculosis but also things like Chlamydia trachomatis, Neisseria gonnorrhoeae, Haemophilus influenzae are other possibilities.
  • Inflammation adjacent to the renal tract – e.g. appendicitis
  • Renal calculi or even just the presence of an indwelling catheter may be associated with pyuria by itself
  • Bladder carcinoma

Also something to think about – situations where uropathogens are grown but the microscopy shows no evidence of inflammation.  Our comment on such urines looks like this:

A neutropenic patient won’t make pyuria either during an infection.

2.       Discuss Pyelonephritis

Clinical Epidemiology,  Pathogens,  Laboratory diagnosis and treatment-  examine your standard treatment guideline sections (adults and children). See above and your textbook.

3.       What are the defining characteristics of Enterobacteriaceae?

Seven common characteristics- a nice Microbeonline summary.  Essential knowledge– Gram negative rods, facultative anaerobes (what does that mean btw?), oxidase negative, convert nitrate to nitrite (hence the urinalysis test for nitrite)

4.       Discuss gentamicin resistance mechanisms in Gram negative bacteria.

Review the RCPA video tutorial on Gram negative resistance.

  • Reduced uptake or decreased cell permeability – e.g. in Pseudomonas – causes pan resistance to all a-g drugs.
  • Modification of the antibiotic -aminoglycoside modification enzymes- may affect just one agent – e.g. gentamicin monoresistance due to the AAC(3)-I mechanism.  Basic types include AAC, ANT, or APH.
  • Modification of the target- altered ribosomal binding sites – e.g. mutational change responsible for streptomycin resistance and also  now, very topical, due to discovery of ribosomal methylases that are carried in multi-resistant GNB that have NDM metallobetalactamases- causes pan-resistance to all agents.

5.       Discuss indications for urine culture and how the specimen should be collected and handled.

In basic terms, collect urine cultures only from patients who have compatible symptoms for UTI (which may include autonomic dysreflexia in paralysed patients).

Asymptomatic bacteriuria becomes highly prevalent in older people and may also be associated with pyuria. Collecting a culture will lead to a false positive result and unnecessary treatment.  Specimens need to be kept refrigerated (4degC)  and set up within 24 hours. Alternatively a container with boric acid can be used for collection- this will prevent overgrowth for up to 24 hours – it is bacteriostatic.

N.B. Avoid collecting samples from indwelling catheters unless the catheter has been in for less than 24 hours.

Specimen collection for urine – also a potential topic to consider – we do not recommend perineal or penile cleansing – foreskin should be retracted and vulva parted for MSU. Bailey and Scott text on urine collection for MSU is not correct- certainly don’t use antiseptic on the perineum prior to collection!

About mdjkf

Microbiologist and Infectious Diseases Physician
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