MRSA Rate

Concept

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Theoretically, Methicillin-Resistant Staphylococcus aureus (MRSA) Rate is an epidemiological measure that quantifies the frequency of MRSA infections or newly identified MRSA cases within a defined healthcare population over a specified period. It is a standard healthcare-associated infection (HAI) surveillance metric used to monitor antimicrobial resistance, evaluate infection prevention strategies and assess healthcare quality. The concept is founded on incidence rate estimation and exists to provide a standardised measure that enables comparison across healthcare providers and reporting periods.

Mathematically, the MRSA Rate is represented as the ratio of confirmed MRSA cases to an appropriate denominator, most commonly patient-days, admissions or patient encounters, multiplied by a standard scaling factor. Many surveillance programmes report MRSA bloodstream infections per 10,000 patient-days, while others use admissions or patient-days depending on reporting requirements.

In practice, MRSA Rates are calculated using microbiology laboratory results linked with hospital activity data. Standardised case definitions are applied before rates are monitored longitudinally, benchmarked against national surveillance programmes and incorporated into infection prevention, antimicrobial stewardship and health economic evaluations.

Purpose

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Used to monitor methicillin-resistant Staphylococcus aureus infections, evaluate infection prevention programmes, assess antimicrobial stewardship initiatives, benchmark healthcare performance and estimate the economic burden of antimicrobial resistance.

Mathematical Formulae

Primary Formula

MRSA Rate = (Number of MRSA Cases � Total Patient-Days) ? 10,000

Supporting Formulae

MRSA Rate per 1,000 Admissions = (Number of MRSA Cases � Total Admissions) ? 1,000

MRSA Percentage = (Number of MRSA Cases � Number of Patients at Risk) ? 100

Related Mathematical Methods

• Incidence rate
• Incidence proportion
• Rate standardisation
• Poisson regression
• Time series analysis
• Statistical process control

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Example

A hospital records 9 laboratory-confirmed MRSA bloodstream infections during one quarter in which patients accumulate 62,500 patient-days.

MRSA Rate = (9 � 62,500) ? 10,000

= 1.44 MRSA infections per 10,000 patient-days

The hospital therefore reports an MRSA Rate of 1.44 per 10,000 patient-days, supporting comparison with historical performance and national infection surveillance benchmarks.

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Excel Implementation

Function	Example Formula	Health Economics Application
COUNTIF	=COUNTIF(B2:B500,"MRSA")	Count confirmed MRSA cases.
SUM	=SUM(C2:C92)	Calculate total patient-days.
COUNTIF/SUM	=(COUNTIF(B2:B500,"MRSA")/SUM(C2:C92))*10000	Calculate the MRSA Rate per 10,000 patient-days.
AVERAGE	=AVERAGE(D2:D13)	Calculate the average monthly MRSA Rate.
IF	=IF(E2>2,"Above Benchmark","Within Benchmark")	Compare MRSA Rates with organisational infection control targets.

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VBA (Optional)

Automate calculation of MRSA rates across hospital units and reporting periods while generating antimicrobial resistance surveillance dashboards.

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Sources

• Centers for Disease Control and Prevention. National Healthcare Safety Network (NHSN) Patient Safety Component Manual.
• World Health Organization. Global Antimicrobial Resistance and Use Surveillance System (GLASS).
• European Centre for Disease Prevention and Control. Surveillance of Antimicrobial Resistance in Europe.
• NICE. Healthcare-Associated Infections: Prevention and Control.
• Drummond MF, Sculpher MJ, Claxton K, Stoddart GL, Torrance GW. Methods for the Economic Evaluation of Health Care Programmes.
• Briggs A, Claxton K, Sculpher M. Decision Modelling for Health Economic Evaluation.