Zar Matrix • CDI Severity Predictor
CDI Score
0
Mild-Moderate Disease
A Zar score ≥ 2 was originally used to justify the choice of Vancomycin over Metronidazole in early CDI guidelines.
Guidelines & Evidence
Verified
Last Review: 2026-07-17
When to Use
When to Use
Prognostic stratification of patients with *Clostridioides difficile* infection (CDI)
To identify patients who require "Severe" treatment regimens (Oral Vancomycin/Fidaxomicin over Metronidazole)
Standardized clinical assessment for initial management in the inpatient setting
Philosophy of Zar
The Zar score is designed to identify "Severe" disease, which carries a much higher risk of treatment failure with Metronidazole.
How it Works
The 6 Severity Item (1 pt each)
Age > 60 years.
Temperature > 38.3 °C (101 °F).
Albumin < 2.5 mg/dL.
WBC > 15,000 cells/mm³.
Endoscopic evidence (Pseudomembranes).
Admitted to the ICU.
Risk Grouping
| 0–1 Points | Mild-to-Moderate Disease |
| ≥ 2 Points | Severe Disease |
Clinical Pearls
Metronidazole vs. Vancomycin
The Zar score was the central tool in the landmark trial that proved Oral Vancomycin was superior to Metronidazole in "Severe" CDI. In the Severe group, Vancomycin achieved a cure rate of 97% compared to only 76% for Metronidazole.
The Albumin Marker
Hypoalbuminemia ($<2.5$) in CDI is a marker of severe "protein-losing enteropathy" caused by the TcdA and TcdB toxins. This state indicates significant mucosal integrity failure and high risk of systemic toxic shock.
Clinical Pearls
Presence of pseudomembranes on sigmoidoscopy is 100% pathognomonic for severe disease
The threshold for severe disease (Score ≥ 2) has a sensitivity of ~80% for predicting treatment failure
In modern IDSA 2021 guidelines, Vancomycin or Fidaxomicin is now often the first choice for all categories, but Zar remains a valuable tool for risk-stratifying those who need intensive monitoring
Next Steps
Management Action (Historical Context)
01
Score ≥ 2 (Severe): Start Oral Vancomycin 125mg QID; Monitor for colonic ileus.
02
Score < 2 (Mild): Oral Metronidazole 500mg TID was previously considered sufficient.
03
Fulminant Disease: Defined as Severe + Shock/Ileus/Megacolon; Start Vancomycin 500mg QID + IV Metronidazole.
Complementary Scoring
ATLAS Score (CDI Mortality)
IDSA/SHEA Severity Criteria
Faecal Calprotectin Interpreter
The Evidence
The Landmark Trial
A comparison of vancomycin and metronidazole for the treatment of Clostridium difficile-associated diarrhea, stratified by disease severity.
Zar FA et al. • Clinical Infectious Diseases. 2007;45(3):302-7. The study that established the Zar score and changed clinical practice globally.
View Source• . ;
Origins & History
Fred A. Zar
Developed by Dr. Fred Zar and his team at the University of Illinois at Chicago. At the time of publication, many clinicians believed Vancomycin was unnecessary; Zar's study provided the objective proof that patient severity should dictate drug choice.
Last Comprehensive Review: 2026-07-17