How to diagnose and treat a urinary tract infection with midstream urine (MSU) culture?

From the Guidelines

To diagnose and treat a urinary tract infection (UTI) with midstream urine (MSU) culture, collect a midstream urine sample after cleaning the genital area with mild soap and water, and send it for culture and sensitivity testing within two hours or refrigerate it if delayed. While awaiting results, empiric treatment can begin for symptomatic patients. The most recent and highest quality study 1 suggests that first-line treatment typically includes nitrofurantoin, trimethoprim-sulfamethoxazole (TMP-SMX), or fosfomycin. For complicated UTIs or pyelonephritis, consider fluoroquinolones like ciprofloxacin or cephalosporins. Once culture results return, therapy should be adjusted based on the identified pathogen and its antibiotic sensitivities. Patients should increase fluid intake and can use phenazopyridine to relieve dysuria. UTIs occur when bacteria, most commonly E. coli, ascend the urethra and colonize the bladder, causing inflammation and the classic symptoms of dysuria, frequency, and urgency.

Key Points for Diagnosis and Treatment

• Collect midstream urine sample after cleaning the genital area with mild soap and water
• Send sample for culture and sensitivity testing within two hours or refrigerate it if delayed
• Start empiric treatment for symptomatic patients while awaiting results
• Adjust therapy based on culture results and antibiotic sensitivities
• Increase fluid intake and consider phenazopyridine for dysuria relief

Considerations for Complicated UTIs

• Consider fluoroquinolones or cephalosporins for complicated UTIs or pyelonephritis
• Be aware of patient risk factors such as underlying medical conditions, pregnancy, or immunosuppression
• Avoid classifying patients as "complicated" unless they have underlying structural or functional abnormalities of the urinary tract or immune suppression 1

Prevention of Recurrent UTIs

• Consider self-start antibiotic therapy for reliable patients with recurrent UTIs
• Avoid treatment of asymptomatic bacteriuria to prevent antimicrobial resistance
• Use nitrofurantoin as a first-line agent for re-treatment when possible 1

From the FDA Drug Label

To reduce the development of drug-resistant bacteria and maintain the effectiveness of sulfamethoxazole and trimethoprim oral suspension and other antibacterial drugs, sulfamethoxazole and trimethoprim oral suspension should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. Urinary Tract Infections For the treatment of urinary tract infections due to susceptible strains of the following organisms: Escherichia coli, Klebsiella species, Enterobacter species, Morganella morganii, Proteus mirabilis and Proteus vulgaris It is recommended that initial episodes of uncomplicated urinary tract infections be treated with a single effective antibacterial agent rather than the combination

The diagnosis of a urinary tract infection (UTI) typically involves a midstream urine (MSU) culture to identify the causative organism.

• Treatment of UTI with trimethoprim/sulfamethoxazole (PO) should be based on culture and susceptibility information.
• The usual adult dosage for UTI is 4 teaspoonfuls (20 mL) every 12 hours for 10 to 14 days 2.
• For children, the recommended dose is 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, given in two divided doses every 12 hours for 10 days 2. Key points:
• Use a single effective antibacterial agent for initial episodes of uncomplicated UTI.
• Consider culture and susceptibility information when selecting or modifying antibacterial therapy.
• Adjust dosage according to renal function and patient weight. 2 2

From the Research

Diagnosing Urinary Tract Infections with Midstream Urine (MSU) Culture

• The MSU culture is considered the gold standard for confirming urinary tract infections (UTIs) 3.
• However, the routine MSU culture may fail to detect a variety of bacterial species, including recognized uropathogens, and may be unsuitable for excluding UTI in patients with lower urinary tract symptoms (LUTS) 3.
• A study found that the MSU culture was unable to discriminate between patients and controls, whereas genomic analysis of urine enriched by centrifugation discriminated between the groups, generating a more accurate understanding of species richness 3.

Diagnostic Tests for UTIs

• The nitrite test has a high positive predictive value (96%) and specificity (94%) for diagnosing UTIs 4.
• A negative nitrite test with a positive leucocyte-esterase (LE) test also has a high positive predictive value (79%) and sensitivity (82%) 4.
• Dipstick urinalysis is a popular diagnostic test, but its results must be interpreted in the context of the patient's pretest probability based on symptoms and characteristics 5.
• Microscopic urinalysis is comparable to dipstick urinalysis as a screening test, and bacteriuria is more specific and sensitive than pyuria for detecting UTIs 5.

Treatment of UTIs

• The first-line treatments for UTIs include nitrofurantoin, fosfomycin, and trimethoprim-sulfamethoxazole (when resistance levels are <20%) 5.
• Amoxicillin-clavulanate is not as effective as ciprofloxacin for the treatment of acute uncomplicated cystitis, even in women infected with susceptible strains 6.
• Resistance to fluoroquinolones, beta-lactams, and trimethoprim-sulfamethoxazole is increasing, but most uropathogens still display good sensitivity to nitrofurantoin 5.

Reducing Contamination in MSU Cultures

• A study found that the use of a funnel urine-collection system and silver-impregnated wipe did not reduce urine-culture contamination in adult midstream clean-catch urine cultures in the emergency department 7.
• The contamination rate in the control group (30%) was markedly lower than the historical ED contamination rate (40%) at the study site 7.