Clinical issues - distinction between urinary tract infections and asymptomatic bacteriuria
Definitions
- Asymptomatic bacteriuria (ASB) = significant bacteriuria (>=10^8 CFU/L, i.e. >=10^5 CFU/mL) on an appropriately collected specimen WITHOUT urinary symptoms
- Pyuria may be present - *pyuria does not make it an infection*
- UTI = bacteriuria PLUS attributable symptoms or signs
Why this matters more in geriatrics than almost anywhere
- *"Positive urine culture" is the single commonest reason an older person is given an unnecessary antibiotic in Australia*
- The reasoning error: an older person is confused -> urine is dipped -> it is positive -> UTI is diagnosed -> the real cause (drug, retention, sepsis elsewhere, stroke, subdural, MI) is missed
- Consequences: C. difficile, resistance, AKI, drug interactions, delirium from the antibiotic, anchoring and diagnostic delay
Symptoms that define a UTI
- Localising urinary: new dysuria, frequency, urgency, suprapubic pain or tenderness, visible haematuria, new or worsening incontinence that is genuinely new
- Upper tract: loin pain, costovertebral angle tenderness, rigors, fever
- Systemic: fever >=38 degrees C, rigors, hypotension with no other source
- *Cloudy or malodorous urine is NOT a symptom of infection* - it reflects concentration and diet
Prevalence of asymptomatic bacteriuria
| Group | Prevalence |
|---|---|
| Healthy young women | ~1-5% |
| Pregnancy | ~2-10% |
| Women >70 in the community | ~10-20% |
| Residential aged care - women | ~25-50% |
| Residential aged care - men | ~15-40% |
| Long-term indwelling catheter | ~100% - bacteriuria is universal after ~30 days |
| Diabetes, spinal cord injury, haemodialysis | High |
- The base rate is the point: at a prevalence of 50%, a positive culture in a delirious nursing home resident tells you almost nothing about the cause of the delirium
Pathogenesis and predisposing factors
- Colonisation of the urinary tract without mucosal invasion or inflammatory injury
- Predisposing factors in older people
- Incomplete bladder emptying - prostatic obstruction, detrusor underactivity, prolapse, constipation
- Oestrogen deficiency -> loss of vaginal lactobacilli -> inc pH -> colonisation with Enterobacterales
- Catheters and instrumentation -> biofilm
- dec functional status, incontinence, immobility, diabetes
- Organisms: E. coli predominates in both ASB and UTI; older and catheterised patients also grow Proteus, Klebsiella, Enterococcus, Pseudomonas, Candida (Candida in urine is almost always colonisation)
- ASB may be protective - eradicating it is associated with a higher rate of subsequent symptomatic UTI with more resistant organisms
The rule: decide clinically BEFORE sending the urine
- *Do not send a urine culture unless a positive result would change management* - i.e. unless there are attributable symptoms
- Do not dipstick asymptomatic older patients - a negative dipstick is useful to exclude; a positive one generates a problem you cannot unsee
Interpreting the dipstick
- High negative predictive value: negative nitrites AND negative leucocyte esterase makes UTI unlikely
- Poor positive predictive value in the elderly - pyuria is present in >90% of ASB and in almost all catheterised patients
- Nitrites require Enterobacterales and several hours' bladder dwell time - a negative nitrite does not exclude enterococcal or pseudomonal infection
Specimen collection
- Midstream, clean-catch; in a catheterised patient, replace the catheter and sample from the new one, never from the bag
- Mixed growth or multiple organisms usually means contamination - repeat rather than treat
The delirious older patient with a positive urine - how to think
1. Is there a localising urinary symptom or sign, or unexplained fever/rigors/loin pain?
- If NO, this is asymptomatic bacteriuria and it is not the explanation
2. Search for the real cause - drugs, pain, constipation, urinary retention, hypoxia, electrolytes, MI, stroke, subdural, infection elsewhere
3. If the patient is septic with no other source, treat as UTI while continuing to look
4. Review at 48-72 h - if they have not improved on antibiotics, the urine was not the cause
When bacteriuria IS worth finding and treating
- Pregnancy - screen at the first antenatal visit; treatment prevents pyelonephritis and preterm birth
- Before an invasive urological procedure that breaches the urothelium (TURP, ureteroscopy with stone manipulation, biopsy) - risk of bacteraemia and sepsis
- Renal transplant in the first few months is debated; most other groups - diabetes, the elderly, catheterised patients, spinal injury, pre-arthroplasty - should NOT be screened or treated
Asymptomatic bacteriuria - management
A. Asymptomatic bacteriuria
- Do not treat, outside pregnancy and pre-urological procedures
- Do not re-culture to document clearance
- Address reversible contributors instead: constipation, retention, atrophic vaginitis (topical oestrogen), fluid intake, catheter removal
Symptomatic UTI in an older adult
B. Symptomatic UTI in an older adult
- Uncomplicated cystitis (non-pregnant, no upper tract or systemic features)
- Trimethoprim 300 mg orally at night for 3 days, OR nitrofurantoin 100 mg orally 6-hourly for 5 days
- *Avoid nitrofurantoin if eGFR <45* - inadequate urinary concentration and inc toxicity
- Cefalexin where resistance or intolerance dictates
- Men: 7 days (treat as complicated); consider prostatic involvement
- Trimethoprim 300 mg orally at night for 3 days, OR nitrofurantoin 100 mg orally 6-hourly for 5 days
- Pyelonephritis / urosepsis
- Blood and urine cultures, then IV gentamicin + amoxicillin (or ampicillin), or ceftriaxone
- Gentamicin: dose on lean body weight and renal function, monitor levels, limit duration - the classic geriatric prescribing trap
- Total 10-14 days; image (CT or ultrasound) if obstruction, stone, abscess or slow response
- Catheter-associated UTI
- Replace the catheter before starting antibiotics; remove it if at all possible
- 7 days if prompt response, up to 14 if delayed
- Review against culture and narrow - and stop if the diagnosis turns out to be something else
Recurrent UTI - prevention without antibiotics first
C. Recurrent UTI - prevention without antibiotics first
- Topical vaginal oestrogen - good evidence in postmenopausal women
- Adequate fluid intake; treat constipation; assess and treat incomplete emptying (post-void residual)
- Methenamine hippurate is a reasonable non-antibiotic option
- Cranberry products: weak and inconsistent evidence; do not rely on them
- Antibiotic prophylaxis only after non-antibiotic measures fail, with a planned review date
- Remove the indwelling catheter - substitute intermittent catheterisation or a continence plan
Antimicrobial stewardship framing
D. Antimicrobial stewardship framing
- Document the indication and a stop date on the chart
- Use the local antibiogram; review at 48-72 h with culture results
- *If the patient is not better after 48-72 h of appropriate antibiotics, the diagnosis is wrong* - go back to the differential
Associations
- Delirium - misattributed to bacteriuria more often than it is caused by UTI
- Indwelling catheters, urinary retention, prostatic disease, pelvic organ prolapse
- Atrophic vaginitis and oestrogen deficiency
- Diabetes, immobility, faecal incontinence, functional dependence
- C. difficile infection, multidrug-resistant organisms (ESBL, VRE) - consequences of over-treatment
- Antibiotic adverse effects in the elderly: AKI, ototoxicity, delirium, drug interactions (trimethoprim with ACEi/ARB or spironolactone -> hyperkalaemia; trimethoprim raises creatinine without changing GFR)
Natural history of bacteriuria
- Asymptomatic bacteriuria in older adults is transient and recurrent, and does NOT cause renal damage, hypertension or increased mortality
- Treating it does not reduce symptomatic UTI, hospitalisation or death - it increases resistance and adverse events
- Organisms come and go spontaneously; ~1/3 clear without treatment and ~1/3 of cleared patients are re-colonised within weeks
- Untreated symptomatic UTI in an older person can progress rapidly to bacteraemia and septic shock - the skill is discriminating the two, not avoiding antibiotics altogether
- Urosepsis has a high mortality in the frail elderly - treat decisively when the clinical picture supports it, and de-escalate on cultures
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