Asymptomatic Bacteriuria

Asymptomatic bacteriuria is the isolation of bacteria in a urine culture at counts of at least 10^5 CFU/mL (women: two consecutive specimens; men: one clean catch specimen; catheter specimen: one specimen at 10^5 CFU/mL) in a person without urinary tract symptoms.

• Prevalence: about 1 to 5 percent of healthy premenopausal women; 4 to 19 percent of older women; 15 to 50 percent of long term care residents; 3 to 10 percent of pregnant women; up to 80 percent in long term catheterised patients and about 25 to 70 percent in those with spinal cord injury and intermittent catheterisation; 10 to 20 percent of diabetics (female).
• It is a state of colonisation, not infection. Protective colonisation with low virulence strains can prevent symptomatic infection.
• In most people, treatment brings no benefit and causes harm: adverse drug effects, Clostridioides difficile infection, selection of antibiotic resistant organisms, loss of protective colonisation, and increased subsequent symptomatic infection.
• Pyuria and a positive dipstick are common in asymptomatic bacteriuria and do not indicate infection; cloudy or offensive urine, delirium or falls alone in an older adult are not reliable indicators of urinary tract infection.
• Only two groups benefit from screening and treatment: pregnant women and people before urological procedures where mucosal bleeding is expected (such as transurethral resection of the prostate).

• By definition, no symptoms: no dysuria, frequency, urgency, suprapubic pain, fever, loin pain or new incontinence.
• Often discovered incidentally on a dipstick or culture sent for another reason.
• In pregnancy: untreated asymptomatic bacteriuria carries an up to 20 to 40 percent risk of progression to acute pyelonephritis, plus associations with preterm birth and low birth weight.
• Assess carefully before attributing non-specific features (for example confusion) to bacteriuria; look for other causes (dehydration, drugs, sepsis from another source, hypoxia, retention, constipation, pain).

• Midstream urine culture with at least 10^5 CFU/mL of a single uropathogen (women: confirm on two consecutive samples collected at least 24 hours apart) without symptoms.
• Do not send urine samples in asymptomatic patients, except for the indicated groups (pregnancy, pre-urological procedures with mucosal trauma, renal transplant in first months some centres).
• Urinalysis in older adults with non-specific features is unreliable: avoid dipstick for diagnosis in over 65s and in catheterised patients; treat only if systemic features of infection (fever above 38 C, rigors, haemodynamic instability, suprapubic or loin pain, new onset urgency or dysuria).
• In pregnancy: culture at the first antenatal visit (ideally 12 to 16 weeks); repeat if positive after treatment.

• Do not treat in: non-pregnant premenopausal women, diabetic women, older people, residents of long term care, patients with indwelling or intermittent catheters (including spinal cord injury), kidney transplant recipients beyond 1 month, neutropenic patients unless clinical features, and patients with urinary diversions or those before non-urological surgery.
• Treat in pregnancy (screen at 12 to 16 weeks):
  • Nitrofurantoin 100 mg modified release orally twice daily for 7 days (avoid in the first trimester when alternatives exist and at 38 weeks onwards and in G6PD deficiency).
  • Cefalexin 500 mg orally twice daily or three times daily for 7 days.
  • Amoxicillin 500 mg orally three times daily for 7 days if sensitive.
  • Fosfomycin 3 g single dose.
  • Avoid trimethoprim in the first trimester, tetracyclines and fluoroquinolones.
  • Repeat culture 1 to 2 weeks after completing treatment, and then periodically during the pregnancy (every 4 weeks, or at each antenatal visit) because recurrence occurs in 20 to 30 percent; consider low dose prophylaxis (cefalexin 250 mg or nitrofurantoin 50 to 100 mg at night) after a second episode.
• Treat before urological procedures that breach the mucosa: culture directed antibiotic started shortly before the procedure (single dose or short course of 1 to 3 doses; for instance, a single oral dose of fosfomycin 3 g 3 hours before, or culture directed intravenous antibiotics at induction), and stop after the procedure.
• Catheter patients: do not culture or treat; when a long term catheter is removed or replaced and symptoms are present, send a sample from the new catheter.
• Education: bacteria in the urine without symptoms is not harmful; patients and carers should resist demands to "treat the urine".
• Follow up: no routine follow up except in pregnancy.