At a glance
- A test should answer a management question.
- Do not culture urine without compatible symptoms except in pregnancy or before selected urologic procedures.
- C. difficile testing requires compatible diarrhea and no better explanation.
- Superficial wound cultures often identify colonizers rather than pathogens.
- Positive tests can cause harm when pretest probability is low.
Objectives
- 1.Explain how diagnostic stewardship supports antimicrobial stewardship.
- 2.Choose when to send urine cultures, blood cultures, C. difficile tests, respiratory panels, and wound cultures.
- 3.Interpret positive tests in the context of colonization versus infection.
- 4.Recognize test-related cascades that lead to unnecessary antibiotics.
- 5.Use pretest probability to decide whether testing is helpful.
Key concepts
Testing should change management
Diagnostic stewardship means ordering the right test for the right patient at the right time and acting appropriately on the result. A test that will not change management can still create harm through false positives, incidental colonization, antibiotic exposure, isolation, cost, and delayed diagnosis of the real problem.
Self-check
Which is the best diagnostic stewardship reason not to send a test?
Select one answer to view the rationale and peer response data.
Urine cultures are overused
Cloudy urine, foul smell, pyuria, and bacteriuria do not diagnose UTI without compatible symptoms. Asymptomatic bacteriuria should not be treated except in pregnancy and before selected invasive urologic procedures. Testing low-probability patients creates antibiotic pressure and C. difficile risk.
Self-check
An afebrile nursing-home resident with a chronic Foley has cloudy urine but no new symptoms. Best approach?
Select one answer to view the rationale and peer response data.
C. difficile testing requires the right stool
Test patients with clinically significant unexplained diarrhea, usually at least three unformed stools in 24 hours, and avoid testing formed stool or laxative-associated diarrhea when another explanation is clear. NAAT detects toxigenic potential and can identify colonization; toxin testing improves specificity depending on local algorithms.
Self-check
Which patient is the best candidate for C. difficile testing?
Select one answer to view the rationale and peer response data.
Blood cultures should match bacteremia risk
Blood cultures are high value in sepsis, endocarditis concern, meningitis, vertebral osteomyelitis, severe pneumonia, complicated pyelonephritis, immunocompromise, intravascular catheter infection, or before antibiotics in serious infection. They are low yield in uncomplicated cellulitis or cystitis.
Self-check
Which scenario most clearly warrants blood cultures before antibiotics?
Select one answer to view the rationale and peer response data.
Respiratory panels require actionability
Respiratory viral panels can support isolation, antiviral decisions, antibiotic de-escalation, and outbreak control, but broad testing in low-risk patients may not change care. A positive viral test also does not fully exclude bacterial coinfection when the clinical syndrome suggests it.
Self-check
A broad respiratory PCR is most useful when the result will affect isolation, antivirals, antibiotics, or disposition. Which case best fits?
Select one answer to view the rationale and peer response data.
Superficial wound swabs mislead
Open wounds are colonized. Superficial swabs often recover colonizers and can drive unnecessary broad antibiotics. When culture is needed, obtain deep tissue or operative specimens after cleaning/debridement, especially for diabetic foot infection or deep surgical infection.
Self-check
A chronic diabetic foot ulcer without systemic signs is superficially swabbed and grows MRSA, Enterococcus, and Pseudomonas. What is the main interpretation problem?
Select one answer to view the rationale and peer response data.
Positive molecular tests can represent colonization
Highly sensitive NAATs can detect colonization, prolonged shedding, or nonviable organisms. Interpret results in context. Examples include C. difficile NAAT in colonized patients, respiratory viral PCR after recent infection, and multiplex wound panels from nonsterile sites.
Self-check
A hospitalized patient on laxatives has formed stool sent for C. difficile NAAT, which is positive. What is the best interpretation?
Select one answer to view the rationale and peer response data.
Pretest probability controls false positives
Even a good test performs poorly when used in patients with very low pretest probability. Low-value testing creates false positives that trigger antibiotic cascades. This principle is especially important for Lyme testing, urine cultures, fungal biomarkers, and broad multiplex panels.
Self-check
Why is Lyme serology discouraged for chronic nonspecific fatigue in a non-endemic area without exposure risk?
Select one answer to view the rationale and peer response data.
Diagnostic stewardship includes stopping tests
Duplicate daily blood cultures, repeated C. difficile tests, repeated urine cultures after clinical improvement, and broad serologic panels can create noise. Stop testing once the clinical question is answered unless the result will change management.
Self-check
A patient with E. coli pyelonephritis improves rapidly on active therapy. Repeat urine culture is ordered to prove cure. Best response?
Select one answer to view the rationale and peer response data.
Good stewardship is collaborative
The best diagnostic stewardship uses clinicians, microbiology labs, pharmacists, infection prevention, and IT. Examples include urine culture reflex criteria, C. difficile testing algorithms, blood culture contamination reduction, antibiogram education, and comments that help clinicians interpret results.
Self-check
Which intervention best represents diagnostic stewardship at the system level?
Select one answer to view the rationale and peer response data.
Evidence & guidelines
Guidelines
- IDSA Asymptomatic Bacteriuria GuidelineInfectious Diseases Society of America, 2019.Essential reference for when not to culture or treat bacteriuria.
Reviews
- Diagnostic stewardship: leveraging the laboratory to improve antimicrobial useMorgan DJ, Malani P, Diekema DJ. JAMA. 2017;318(7):607-608. PMID: 28719668. DOI: 10.1001/jama.2017.8531.Concise foundational article connecting diagnostic ordering, lab systems, and antimicrobial use.
Pearls
- PearlA positive culture from the wrong patient is not helpful information.
- PearlDo not let a test substitute for a syndrome.
- PearlColonization is common in urine, wounds, airways, and stool.
- PearlThe best test may be no test when pretest probability is low.