A cardiac stress test is a non-invasive medical procedure that assesses how well your heart functions under physical exertion. By gradually increasing the workload on your heart, physicians can observe its electrical activity, blood pressure response, and overall capacity to pump blood. This evaluation is a powerful tool for diagnosing potential blockages in the coronary arteries and determining your overall cardiorespiratory fitness level. The primary measurement used to quantify your performance during this test is the Metabolic Equivalent of Task, or MET score.
Defining the Metabolic Equivalent of Task (MET)
The Metabolic Equivalent of Task, or MET, is a standardized unit that represents the energy expenditure of physical activities. By definition, one MET is the amount of oxygen your body consumes while sitting quietly at rest. This value is scientifically fixed at approximately 3.5 milliliters of oxygen consumed per kilogram of body weight per minute (3.5 ml O2/kg/min).
The MET system allows medical professionals to compare the intensity of various activities and the exercise capacity of different individuals on a universal scale. For example, an activity requiring 4 METs demands four times the energy and oxygen consumption as the body at rest.
Calculating METs During a Cardiac Stress Test
A patient’s MET score during a stress test is determined by the maximum workload they are able to achieve on the treadmill. This workload is precisely controlled by increasing the speed and incline of the treadmill in set increments. Each stage of this protocol is standardized to correlate with a specific MET level.
In the Bruce protocol, the first stage typically begins at 1.7 miles per hour with a 10% incline, which corresponds to an estimated workload of 5 METs. The test progresses in three-minute stages, with the treadmill’s speed and grade increasing incrementally to demand greater oxygen consumption. The maximum MET score achieved is an estimate derived from the final stage completed before the test is stopped due to exhaustion or medical necessity.
Age and Sex-Specific METs Score Benchmarks
The MET score is a range that changes significantly based on a person’s age and sex. Generally, men tend to achieve slightly higher absolute MET scores than women across all age groups, a difference often related to body size and composition.
A score of 12 METs or higher is generally considered excellent for most healthy adults, while a score below 5 METs is often categorized as poor or low functional capacity. A more detailed assessment compares an individual’s result to the performance of others in the same age and sex bracket, typically using percentile data. Performance falling into the 50th percentile is considered average, while a score in the 90th percentile or above represents an excellent fitness level.
The following benchmarks illustrate average (50th percentile) and excellent (90th percentile) scores:
- For a 40-year-old man, average performance is approximately 8.8 METs, while an excellent score is 11.3 METs.
- A 40-year-old woman would have an average score of about 8.1 METs and an excellent score of 10.4 METs.
- The thresholds for a man aged 60 and older are lower, with an average score around 7.4 METs and an excellent score at 10.0 METs.
- For a woman in the 60+ age group, an average MET score is approximately 6.8, and an excellent score is 8.9.
Clinical Significance of Your METs Score
The MET score is a powerful and independent predictor of long-term health and survival. A higher MET score is consistently associated with a more favorable health outlook.
For every 1-MET increase in exercise capacity, there is an approximate 13% reduction in the risk of all-cause mortality and a 15% reduction in the risk of coronary heart disease events. Achieving a score of 10 METs is a particularly important benchmark, as patients who reach this level generally enjoy an excellent prognosis, even if they have other underlying heart conditions. Conversely, an inability to reach 5 METs is a significant indicator of increased risk for future cardiac events.
A physician interprets the MET score alongside other findings from the test, such as any changes on the electrocardiogram (EKG) and the patient’s symptoms. This comprehensive evaluation allows them to determine the final diagnosis and establish an appropriate treatment plan. The MET score provides a tangible measure of the heart’s functional reserve, which is a powerful tool for risk stratification.