14.2 Stewardship Metrics: Days of Therapy (DOT), DDD, and SAAR

Stewardship Metrics: Days of Therapy (DOT), DDD, and SAAR

Accurate, reproducible measurement of antimicrobial consumption is essential for evaluating the impact of stewardship interventions, establishing institutional benchmarks, and identifying targets for quality improvement. Over the past two decades, antimicrobial metrics have evolved from crude financial drug acquisition expenditures to standardized, clinical consumption units, culminating in risk-adjusted national benchmarking models established by the Centers for Disease Control and Prevention (CDC) National Healthcare Safety Network (NHSN).

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Quantitative Consumption Metrics: Definitions and Mechanics

                     COMPARATIVE SPECTRUM OF CONSUMPTION METRICS

      METRIC               PRIMARY UNIT                 KEY STRENGTH                 MAJOR LIMITATION
 ┌──────────────┐   ┌────────────────────────┐   ┌───────────────────────────┐   ┌───────────────────────────┐
 │ Days of      │   │ Calendar day with ≥1   │   │ Unaffected by renal dose  │   │ Double-counts combination │
 │ Therapy (DOT)│   │ dose of specific agent │   │ adjustments; pediatric ok │   │ therapy (overstates LOT)  │
 ├──────────────┤   ├────────────────────────┤   ├───────────────────────────┤   ├───────────────────────────┤
 │ Defined      │   │ Total grams ÷ WHO      │   │ Standardized global       │   │ Renal dose reductions     │
 │ Daily Dose   │   │ assigned adult daily g │   │ comparative metric        │   │ falsely deflate consumption│
 ├──────────────┤   ├────────────────────────┤   ├───────────────────────────┤   ├───────────────────────────┤
 │ Length of    │   │ Calendar day on ANY    │   │ Measures true duration of │   │ Obscures number or breadth│
 │ Therapy (LOT)│   │ systemic antimicrobial │   │ patient exposure          │   │ of agents administered    │
 └──────────────┘   └────────────────────────┘   └───────────────────────────┘   └───────────────────────────┘

1. Days of Therapy (DOT)

• Formal Definition: A Day of Therapy (DOT) represents an aggregate sum of calendar days on which a patient receives at least one dose of a specific systemic antimicrobial agent, irrespective of the dose, dosing frequency, route of administration, or renal dose adjustments.
• Standard Denominators:
  • Days Present (DP): The preferred CDC NHSN denominator. Defined as the total number of unique patients present in a clinical unit for any portion of a calendar day.
  • Patient Days (PD): Traditional hospital census denominator based on midnight bed census.
  • Metric Expression: Expressed as DOT per 1,000 Days Present or DOT per 1,000 Patient Days: $\text{DOT Rate} = \left( \frac{\sum \text{Days of Therapy}}{\sum \text{Days Present}} \right) \times 1,000$
• Clinical Nuances and Advantages:
  • Renal Dose Immunity: If a patient with normal renal function receives cefepime $2\text{ g IV q8h}$ for 5 days, they accumulate $5\text{ DOT}$. If a hemodialysis patient receives cefepime $1\text{ g IV q24h}$ for 5 days, they also accumulate exactly $5\text{ DOT}$. The metric accurately captures that both patients experienced 5 days of pathogen exposure.
  • Pediatric Applicability: Because DOT is independent of milligrams administered, it applies seamlessly to pediatric cohorts where weight-based dosing varies dramatically.
  • Combination Inflation: If a patient receives vancomycin plus piperacillin-tazobactam for 7 days, they accumulate $7\text{ vancomycin DOT} + 7\text{ piperacillin-tazobactam DOT} = 14\text{ total DOT}$.

2. Defined Daily Dose (DDD)

• Formal Definition: Established by the World Health Organization (WHO) Collaborating Centre for Drug Statistics Methodology, the Defined Daily Dose (DDD) is a technical, statistical unit of measurement defined as the assumed average maintenance dose per day for a drug used for its main indication in adults.
• Assigned WHO Reference Values:
  • Meropenem: $3\text{ g}$ (equivalent to $1\text{ g IV q8h}$)
  • Ceftriaxone: $2\text{ g}$ (equivalent to $2\text{ g IV q24h}$ or $1\text{ g IV q12h}$)
  • Levofloxacin: $500\text{ mg}$ (equivalent to $500\text{ mg once daily}$)
  • Cefepime: $4\text{ g}$ (equivalent to $2\text{ g IV q12h}$)
  • Vancomycin (parenteral): $2\text{ g}$ (equivalent to $1\text{ g IV q12h}$)
• Mathematical Calculation: $\text{DDD} = \frac{\text{Total Grams Administered}}{\text{Assigned WHO Standard DDD (grams)}}$ $\text{DDD Rate} = \left( \frac{\sum \text{DDDs}}{\sum \text{Patient Days}} \right) \times 1,000$
• Critical Clinical Limitations:
  • Renal Adjustment Bias: Consider a patient with end-stage renal disease ($CrCl < 10\text{ mL/min}$) treated with meropenem $500\text{ mg IV q24h}$ for 6 days. The patient receives a total of $3.0\text{ g}$ of meropenem. Under the WHO standard ($DDD = 3\text{ g}$), this 6-day course calculates as $\frac{3.0\text{ g}}{3.0\text{ g}} = 1.0\text{ DDD}$. The DDD metric falsely suggests the patient received only 1 day of therapy, understating actual antimicrobial exposure by 83%!
  • Dose Escalation Artifact: In bacterial meningitis, high-dose meropenem ($2\text{ g IV q8h} = 6\text{ g/day}$) for 14 days results in $84\text{ g}$ administered. Calculated as $\frac{84\text{ g}}{3\text{ g}} = 28\text{ DDDs}$, suggesting 28 days of therapy rather than the actual 14 calendar days.
  • Inapplicability in Pediatrics: Fixed adult gram benchmarks cannot be applied to children.

3. Length of Therapy (LOT)

• Formal Definition: Length of Therapy (LOT) represents the number of calendar days that a patient received any systemic antimicrobial agent, regardless of the number of different agents administered.
• Clinical Purpose: LOT directly measures the duration of patient exposure to antimicrobial selection pressure. Unlike DOT, LOT does not double-count combination regimens.

Illustrative Worked Case Comparison

Consider an intensive care unit patient with septic shock secondary to hospital-acquired pneumonia treated with IV vancomycin $1,500\text{ mg q12h}$ PLUS IV cefepime $2\text{ g q8h}$ for 7 days. On Day 8, both agents are discontinued, and the patient is transitioned to oral levofloxacin $750\text{ mg q24h}$ for 3 additional days.

 Day:           1   2   3   4   5   6   7   8   9  10
 Vancomycin:   [X] [X] [X] [X] [X] [X] [X]
 Cefepime:     [X] [X] [X] [X] [X] [X] [X]
 Levofloxacin:                             [X] [X] [X]
 ────────────────────────────────────────────────────────
 Total Days:   ◄─────── 7 Days ────────►   ◄── 3 Days ──►

• Calculation of DOT:
  • Vancomycin DOT: $7\text{ days}$
  • Cefepime DOT: $7\text{ days}$
  • Levofloxacin DOT: $3\text{ days}$
  • Total DOT: $7 + 7 + 3 = 17\text{ DOT}$
• Calculation of LOT:
  • The patient received at least one antimicrobial on Days 1 through 10.
  • Total LOT: $10\text{ calendar days}$
• Calculation of DDD:
  • Vancomycin: $3\text{ g/day} \times 7\text{ days} = 21\text{ g} \div 2\text{ g WHO DDD} = 10.5\text{ DDD}$
  • Cefepime: $6\text{ g/day} \times 7\text{ days} = 42\text{ g} \div 4\text{ g WHO DDD} = 10.5\text{ DDD}$
  • Levofloxacin: $0.75\text{ g/day} \times 3\text{ days} = 2.25\text{ g} \div 0.5\text{ g WHO DDD} = 4.5\text{ DDD}$
  • Total DDD: $10.5 + 10.5 + 4.5 = 25.5\text{ DDD}$

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The Standardized Antimicrobial Administration Ratio (SAAR)

While DOT provides an accurate internal count of drug usage, raw DOT rates cannot be directly compared across different hospitals or different clinical units. A tertiary surgical trauma ICU caring for complex septic shock inherently requires higher broad-spectrum antibiotic consumption than a community medical ward. To enable fair, risk-adjusted national benchmarking, the CDC NHSN developed the Standardized Antimicrobial Administration Ratio (SAAR) within the Antimicrobial Use (AU) Option.

Mathematical Formulation

The SAAR is an indirect standardization metric comparing the observed antimicrobial administration to the statistically predicted antimicrobial administration:

$SAAR = \frac{\text{Observed DOT}}{\text{Predicted DOT}}$

• Observed DOT: The actual number of Days of Therapy electronically extracted from the hospital's medication administration record (MAR) and submitted to NHSN for a specified patient care location and drug category.
• Predicted DOT: The number of Days of Therapy statistically predicted to be administered in that specific unit, derived from nationally representative negative binomial multivariable regression models developed by the CDC.

CDC Risk-Adjustment Variables

The NHSN predictive models adjust for intrinsic institutional and patient care location factors, including:

• CDC Location Category: Medical ICU, surgical ICU, mixed medical/surgical ICU, adult medical ward, surgical ward, step-down unit, oncology ward, etc.
• Facility Characteristics: Hospital bed size, medical school affiliation (major teaching, graduate teaching, non-teaching), hospital type (general acute care, critical access, military, oncology).
• Diagnostic Testing Capabilities: Availability of on-site rapid blood culture molecular diagnostics.
• Inpatient Acuity and Infection Rates: Facility-level hospital-onset C. difficile infection (HO-CDI) labID incidence rates and average length of stay.

The 16 NHSN SAAR Antimicrobial Categories

NHSN stratifies antimicrobial agents into specific SAAR reporting categories (separated across adult, pediatric, and neonatal models). The core Adult Inpatient SAAR Categories include:

1. Broad-Spectrum Antibacterial Agents Used for Hospital-Onset Infections: Agents typically reserved for nosocomial or multi-drug resistant pathogens: carbapenems (meropenem, imipenem, ertapenem), cefepime, piperacillin-tazobactam, ceftazidime, aztreonam, novel beta-lactamase inhibitor combos, and aminoglycosides.
2. Broad-Spectrum Antibacterial Agents Used for Community-Acquired Infections: Agents primarily utilized for community-onset sepsis and pneumonia: third-generation cephalosporins (ceftriaxone, cefotaxime), ampicillin-sulbactam, and respiratory fluoroquinolones (levofloxacin, moxifloxacin).
3. Anti-MRSA Antibacterial Agents: Targeted agents covering methicillin-resistant S. aureus: vancomycin (IV), daptomycin, linezolid, tedizolid, ceftaroline, dalbavancin, oritavancin.
4. Anti-Pseudomonal Antibacterial Agents: Agents possessing activity against Pseudomonas aeruginosa (cefepime, ceftazidime, piperacillin-tazobactam, meropenem, imipenem, aztreonam, ciprofloxacin, levofloxacin, aminoglycosides, polymyxins).
5. Narrow-Spectrum Beta-Lactam Agents: Targeted, low-collateral-damage agents: cefazolin, cephalexin, ampicillin, amoxicillin, penicillin G, nafcillin, oxacillin.
6. Antibacterial Agents Posing High Risk for Clostridioides difficile Infection: Antimicrobial classes historically possessing the highest propensity for precipitating CDI: 3rd and 4th generation cephalosporins, fluoroquinolones, clindamycin, and carbapenems.

Clinical Interpretation of the SAAR and Confidence Intervals

To interpret a SAAR value correctly, clinicians must evaluate both the point estimate and the associated 95% Confidence Interval (CI):

                         SAAR INTERPRETATION SPECTRUM

   SAAR < 1.0 (Upper CI < 1.0)        SAAR = 1.0        SAAR > 1.0 (Lower CI > 1.0)
◄──────────────────────────────┼───────────────────────┼──────────────────────────────►
  Significantly LOWER Use        Matches Baseline        Significantly HIGHER Use
 • Potential under-treatment    • Normal variation      • Excessive broad-spectrum use
 • Effective de-escalation      • Expected consumption  • Opportunity for prospective
 • Verify data completeness                             audit and stewardship action

1. $SAAR = 1.0$ (or 95% CI includes 1.0): Observed antibiotic administration is consistent with the national predictive baseline. Prescribing reflects expected utilization for that unit type.
2. $SAAR > 1.0$ with 95% CI Lower Bound $> 1.0$: Observed use is statistically significantly higher than predicted by the national model. This serves as an actionable screening alert indicating potential overuse, delayed de-escalation, or empirical broad-spectrum overprescribing, warranting focused prospective audit.
3. $SAAR < 1.0$ with 95% CI Upper Bound $< 1.0$: Observed use is statistically significantly lower than predicted. While often reflecting successful stewardship de-escalation, ASPs must verify that under-prescribing or delayed therapy is not compromising patient safety in high-acuity infections.

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Clinical and Financial Balancing Outcome Metrics

High-performing ASPs avoid measuring consumption in a silo. Optimizing stewardship requires tracking balancing measures—clinical safety indicators that ensure reductions in antimicrobial use do not inadvertently harm patients:

1. Clinical Outcome Metrics

• Hospital-Onset Clostridioides difficile Infection (HO-CDI) Incidence: Measured as the number of laboratory-identified (LabID) hospital-onset CDI events per 10,000 patient days. A direct clinical surrogate of broad-spectrum antimicrobial collateral damage.
• 30-Day All-Cause and Infection-Related Mortality: Ensures that early antibiotic de-escalation or abbreviated therapy durations do not increase patient mortality in bloodstream infections, pneumonia, or sepsis.
• 30-Day Hospital Readmission Rates: Monitors whether patients discharged on streamlined or oral regimens experience treatment failure requiring re-hospitalization.
• Hospital and ICU Length of Stay (LOS): Quantifies recovery velocity and timely resolution of infectious episodes.

2. Financial and Pharmacoeconomic Metrics

• Direct Drug Acquisition Costs: The aggregate expenditure on antimicrobial agents. While easy to track, acquisition costs can be deceptive; switching from IV vancomycin ($10/day) to oral linezolid ($30/day) increases drug acquisition cost but facilitates earlier hospital discharge, saving thousands of dollars in room-and-board charges.
• Total Healthcare Cost Savings (Cost Avoidance): Encompasses prevented adverse drug events (such as avoiding hemodialysis from aminoglycoside-induced AKI), shortened length of stay, and reduced intensive care unit utilization.