What is Absolute Risk Reduction (ARR) and how is it used to evaluate the effectiveness of a treatment?

Understanding Absolute Risk Reduction (ARR)

Absolute Risk Reduction (ARR) is the arithmetic difference between event rates in control and treatment groups, representing the actual percentage of patients who avoid a negative outcome due to treatment, and is the most clinically meaningful measure for evaluating treatment effectiveness.

Definition and Calculation

Absolute Risk Reduction is calculated using the following formula:

• ARR = Control Event Rate (CER) - Experimental Event Rate (EER) 1
• Expressed as a percentage or decimal (e.g., 0.05 or 5%)

For example, in the SPRINT trial data mentioned in the guidelines, the primary outcome rate was 2.19% per year in the standard treatment group and 1.65% per year in the intensive treatment group, yielding an ARR of 0.54% per year 2.

Clinical Significance of ARR

ARR provides several advantages over relative measures:

• Direct clinical relevance: Shows the actual proportion of patients who benefit from treatment
• Accounts for baseline risk: Unlike Relative Risk Reduction (RRR), ARR incorporates the baseline risk of the control group 2
• Enables NNT calculation: ARR allows calculation of Number Needed to Treat (NNT = 1/ARR), which estimates how many patients need treatment to prevent one adverse outcome 2

ARR vs. Relative Risk Reduction (RRR)

ARR provides more clinically useful information than RRR:

• RRR remains constant regardless of baseline risk, potentially overestimating treatment effects 2
• For the same RRR, the ARR will be larger in high-risk populations and smaller in low-risk populations 2
• Example: A treatment that reduces risk from 2% to 1% and another that reduces risk from 40% to 20% both have an RRR of 50%, but their ARRs are very different (1% vs. 20%) 2

Using ARR in Clinical Decision-Making

When evaluating treatment effectiveness:

1. Identify the baseline risk for your patient population
2. Calculate the ARR using trial data
3. Determine the NNT (1/ARR) to understand treatment effort required
4. Consider time horizon - ARR and NNT should always specify the time period 2

Common Pitfalls in ARR Interpretation

• Ignoring time frame: ARR without a specified time period is meaningless 2
• Not considering baseline risk: Treatment benefit varies based on patient's baseline risk 2
• Focusing only on relative measures: Clinical trials often emphasize RRR, which can exaggerate treatment effects 2
• Not considering harms: ARR should be balanced against absolute risk increase of adverse effects 2

Population Impact of ARR

Even a seemingly small ARR can have significant population impact when the condition is common. For example, the SPRINT trial showed an ARR of "only" 0.54% per year with intensive blood pressure treatment, but when applied to the broader population, this could prevent 90,000 cardiovascular events, 62,000 deaths, and 10,100 strokes annually in the United States alone 2.

Conclusion

ARR is essential for translating clinical trial results into meaningful patient care decisions. While relative measures like RRR may appear more impressive, ARR provides the actual magnitude of benefit patients can expect from treatment, making it the preferred metric for evaluating treatment effectiveness in clinical practice.