The Dirty Truth: Are We Preventing C. difficile — Or Just Positive Tests?
Is NHSN measuring actual C. difficile infections? Or are hospitals being held accountable for positive laboratory results?
Under the National Healthcare Safety Network's (NHSN) current surveillance definition, a patient can test positive for toxigenic Clostridioides difficile by PCR, have a negative toxin assay, and have no confirmed clinical diagnosis of CDI. Yet still meet the criteria for a healthcare facility-onset CDI LabID Event.
And these aren't just surveillance statistics. They can have real financial consequences for hospitals.
Healthcare facility-onset CDI LabID Events contribute to publicly reported hospital performance measures used by the Centers for Medicare & Medicaid Services (CMS), including Hospital Value-Based Purchasing (VBP) and the Hospital-Acquired Condition Reduction Program (HACRP).
In other words, a laboratory result that may represent colonization rather than active infection can influence a hospital's performance ratings and Medicare reimbursement.
I have a problem with that.
Not because I oppose standardized surveillance. Having spent years performing NHSN surveillance, I understand the importance of objective definitions and consistent reporting.
But my background in clinical microbiology has taught me something equally important:
Detecting an organism does not necessarily mean it's causing disease.
And when we're holding hospitals accountable for healthcare-associated infections, shouldn't that distinction matter?
C. difficile Testing: When the Tests Don't Tell the Whole Story
Consider a stool specimen with the following results:
C. difficile PCR: Positive
GDH antigen: Negative
Toxin A/B EIA: Negative
Not an unusual finding, as most of you know. PCR is highly sensitive, detecting genetic material from toxigenic C. difficile without establishing whether the organism is causing disease. The negative GDH antigen and toxin assays don't rule out infection either, making clinical assessment essential when interpreting discordant results.
Does the patient have unexplained diarrhea? Were laxatives administered? Could another condition explain the symptoms? Does the treating provider suspect CDI?
NHSN doesn't require those answers to classify a qualifying CDI LabID Event.
And that's where the controversy begins.
What Exactly Is NHSN Measuring?
NHSN introduced CDI LabID Event surveillance in March 2009 to provide a standardized, less labor-intensive approach to monitoring C. difficile across healthcare facilities. Rather than requiring extensive clinical review, NHSN relies on laboratory results and established reporting rules.
A qualifying positive C. difficile laboratory result collected on or after Hospital Day 4 can be classified as a healthcare facility-onset CDI LabID Event, provided the other reporting criteria are met. But that classification doesn't establish that the patient acquired C. difficile during hospitalization. Or that the patient has an active infection.
In fact, NHSN itself describes LabID surveillance as a proxy measure of infection burden, based primarily on laboratory findings rather than clinical evaluation. Its 2026 protocol explicitly states that signs and symptoms are not required for LabID Event reporting.
CDC acknowledges the distinction between laboratory-based surveillance and clinically defined infection. Yet those laboratory-based measures contribute to hospital performance ratings and Medicare reimbursement.
THAT deserves scrutiny.
When C. difficile Testing Algorithm Changes the Outcome
Here's where I find NHSN's definition particularly troubling.
Hospitals don't all test for C. difficile the same way. Some laboratories use PCR alone. Others use multistep algorithms combining PCR, GDH antigen, and toxin testing.
Under NHSN's surveillance rules, the documented sequence of testing—and even the timestamps on the final laboratory report—can determine whether a specimen qualifies as a positive CDI laboratory assay.
Consider three scenarios.
Scenario 1: PCR Followed by Toxin Testing
Test | Result | Time |
PCR | Positive | 1:00 PM |
GDH antigen | Negative | 1:30 PM |
Toxin A/B EIA | Negative | 1:30 PM |
NHSN outcome: Does not qualify as a positive CDI laboratory assay.
The final testing step is negative.
Scenario 2: Simultaneously Documented Testing
Test | Result | Time |
PCR | Positive | 1:00 PM |
GDH antigen | Negative | 1:00 PM |
Toxin A/B EIA | Negative | 1:00 PM |
NHSN outcome: Qualifies as a positive CDI laboratory assay.
With identical timestamps, NHSN considers the testing simultaneous. The positive PCR establishes a qualifying laboratory result.
Same findings. Different timestamps. Different surveillance outcomes.
And here's another commonly used laboratory approach.
Scenario 3: GDH/Toxin First, Followed by Reflex PCR
Test | Result | Time |
GDH antigen | Positive | 1:00 PM |
Toxin A/B EIA | Negative | 1:00 PM |
PCR | Positive | 1:30 PM |
NHSN outcome: Qualifies as a positive CDI laboratory assay.
The discordant GDH-positive/toxin-negative results trigger reflex PCR. Because the final PCR is positive, the specimen qualifies.
Now compare Scenarios 1 and 3.
Both have positive PCR results and negative toxin EIAs.
But in Scenario 1, the specimen does not qualify. In Scenario 3, it does.
The difference? Which test was performed last.
These scenarios follow NHSN's published multistep testing rules. They do not suggest that laboratories should alter timestamps or manipulate testing procedures, and additional NHSN criteria still apply before an eligible assay becomes a reportable LabID Event.
But the underlying concern remains.
Should laboratory workflow and documentation have this much influence over a measure used to evaluate hospital infection prevention performance?
Neither algorithm independently establishes whether a patient has toxin-mediated disease. Yet the testing sequence can influence whether a positive result contributes to hospital-onset CDI surveillance.
When Hospitals Pay the Price for Positive C. difficile Testing
NHSN healthcare facility-onset CDI LabID Events contribute to a hospital's standardized infection ratio (SIR), which is used in federal hospital quality programs. Under Hospital VBP, CDI performance contributes to the Safety Domain and influences overall performance scoring and Medicare payment adjustments. Under HACRP, CDI is one of several measures used to evaluate hospital performance. Hospitals in the worst-performing quartile receive a 1% reduction in applicable Medicare fee-for-service payments.
Of course, a single CDI LabID Event doesn't automatically trigger a penalty. But, multiple measures can.
And that raises a question of fairness: Should hospital reimbursement and public performance ratings be influenced by results that may not represent actual infections?
Are We Measuring Infections—or Testing Practices?
There's another factor we can't ignore: diagnostic stewardship.
Not every patient with loose stools should undergo C. difficile testing. Diarrhea has numerous causes, including medications, laxatives, and underlying medical conditions. Testing patients without appropriate clinical indications increases the likelihood of identifying colonization rather than active disease.
IDSA/SHEA guidelines recommend targeting patients with unexplained, new-onset diarrhea, generally defined as three or more unformed stools within 24 hours.
Now consider two hospitals.
Hospital A has a strong diagnostic stewardship program. Testing is limited to clinically appropriate patients, and unnecessary orders are discouraged.
Hospital B tests a broader population, including patients with alternative explanations for diarrhea.
Hospital B may identify more positive results simply because it tests more patients.
Does that necessarily mean Hospital B has more hospital-acquired CDI?
Of course not.
Differences may reflect true infection rates, patient populations, testing practices, or some combination of these factors. To NHSN's credit, its CDI risk-adjustment methodology accounts for laboratory test type and certain facility characteristics. But risk adjustment cannot determine whether an individual positive result represents colonization or active infection. Nor can it fully eliminate differences in which patients clinicians choose to test.
A hospital's reported CDI performance may reflect more than its infection prevention practices.
Why NHSN Uses LabID Surveillance
To be fair, NHSN has legitimate reasons for relying on laboratory-based surveillance. Standardized reporting reduces subjectivity, improves reproducibility, and limits the substantial workload associated with clinical case review.
But consistency and clinical validity are not the same thing.
A definition can be applied perfectly consistently and still fail to distinguish infection from colonization in individual patients.
Isn't It Time for a Better Measure?
I'm not suggesting NHSN abandon CDI LabID surveillance. Continue collecting laboratory-based data. Continue monitoring trends. Continue using standardized results to identify possible changes in infection burden.
But I believe CDC/NHSN should evaluate a clinically validated measure for hospital performance reporting.
Such a measure could incorporate:
Clinical symptoms: Does the patient have unexplained diarrhea or another presentation consistent with CDI?
Laboratory findings: Which tests were performed, and how should discordant findings be interpreted?
Alternative explanations: Could laxatives, medications, or another medical condition explain the symptoms?
Provider assessment: Does the treating clinician suspect active CDI?
Treatment decisions: Was CDI-directed treatment, such as oral vancomycin or fidaxomicin, clinically indicated or initiated?
No single factor should determine the outcome. Nor should facilities be allowed to exclude events simply because a provider documents colonization or chooses not to treat.
A new performance measure would require objective criteria, validation, and safeguards against inconsistent reporting. And it would undoubtedly introduce challenges. But shouldn't we at least investigate whether a more clinically meaningful measure can be developed without sacrificing the benefits of standardized surveillance?
The Dirty Truth
Here's something I've witnessed during my years in infection prevention: hospitals modifying their C. difficile testing algorithms and implementing additional testing restrictions, sometimes even requiring Infection Prevention approval before a specimen can be tested.
There's nothing inherently wrong with that.
Appropriate diagnostic stewardship is essential. We shouldn't test patients who don't meet established clinical criteria.
But there's a difference between preventing inappropriate testing and preventing a positive result from becoming an NHSN event.
And that's where we need to ask some uncomfortable questions. Are hospitals changing their testing practices to improve diagnostic accuracy and patient care? Or are surveillance definitions and financial incentives influencing which tests are performed?
The answer may be both.
I'm not suggesting that hospitals implementing testing restrictions are necessarily manipulating their infection rates. Many have excellent diagnostic stewardship programs. But when a surveillance measure carries financial consequences, hospitals have incentives to manage not only infections, but also the testing practices that determine which events are counted.
The uncomfortable truth is that a hospital can improve its reported CDI performance without necessarily reducing its actual infection burden.
That's a limitation worth confronting.
We've become extraordinarily good at detecting microorganisms. Molecular diagnostics have transformed clinical microbiology, but our ability to detect an organism doesn't guarantee that it's causing disease. Our surveillance definitions need to recognize that distinction.
I believe CDC/NHSN should seriously investigate a clinically validated approach to measuring hospital-onset CDI for performance reporting. Because ultimately, infection prevention isn't about preventing positive laboratory results. It's about preventing infections.
References
Centers for Disease Control and Prevention. (2026). NHSN Patient Safety Component Manual, Chapter 12: Multidrug-Resistant Organism & Clostridioides difficile Infection Module. View protocol.
Centers for Disease Control and Prevention. NHSN Frequently Asked Questions: MDRO & CDI. View FAQs.
Centers for Disease Control and Prevention. (2012). Vital Signs: Preventing Clostridium difficile Infections. MMWR. Read report.
McDonald, L. C., et al. (2018). Clinical Practice Guidelines for Clostridium difficile Infection in Adults and Children: 2017 Update by SHEA/IDSA. Clinical Infectious Diseases, 66(7), e1–e48. Read guidelines.
Centers for Disease Control and Prevention. 2022 NHSN HAI Rebaseline: CDI LabID Event Risk Adjustment. View model.
Centers for Medicare & Medicaid Services. Hospital Value-Based Purchasing Program. View program.
Centers for Medicare & Medicaid Services. Hospital-Acquired Condition Reduction Program. View program.
