Imagine a doctor trying to describe how well a patient walks. The goal is to be precise. But the tools available today often fail to capture the full picture. This confusion happens because of a system called the International Classification of Functioning, Disability and Health. Doctors are supposed to use specific qualifiers within this system. These qualifiers tell us two different things. One tells us what a person can do. The other tells us what a person actually does.
This distinction matters a lot. Knowing a patient can walk ten feet is different from knowing they walked ten feet yesterday. Yet many clinicians mix these two ideas together. They report on ability and actual performance without clearly separating them. This lack of clarity makes it hard to compare patients across different clinics or hospitals. It also makes it difficult to track progress over time.
The Old Way of Measuring Health
For decades, doctors have relied on their own judgment to rate patient function. They look at a patient and assign a score based on observation. This approach has always been common. However, the ICF system was designed to bring order to this process. It offers a standardized way to talk about health and disability. The problem is that few doctors follow the rules strictly. Most assessments still rely on general impressions rather than specific criteria.
But here's the twist. A recent review found that the situation has not improved much in twenty-five years. The original definitions for capacity and performance were clear when the system launched. Yet, their use in daily practice remains limited and inconsistent. Clinicians often retrofit existing tests to fit the system. They use percentage thresholds or descriptive labels that vary from one doctor to the next. This creates a patchwork of data that lacks true standardization.
A Factory Analogy for Function
Think of the human body as a factory. Capacity is like the machine's potential output. It represents the maximum speed the machine could run if everything worked perfectly. Performance is like the actual production line speed. It accounts for breaks, power outages, or operator fatigue. In medicine, capacity is the potential to move a limb. Performance is the movement seen in real life.
The difference is crucial. A patient might have the strength to lift a weight. But pain or fear might stop them from doing so. The ICF qualifiers are meant to capture this gap. They help doctors understand why a patient is not performing at their full potential. Without these qualifiers, a doctor might think a patient is weak. In reality, the patient might just be in pain.
Researchers looked at five studies published between 2001 and August 2025. They searched for assessments that used the ICF qualifiers correctly. The results were not encouraging. The identified assessments were confined to a narrow range of mobility categories. They mostly covered walking and the use of upper extremities. Other areas of function were largely ignored.
Most studies used heterogeneous approaches to apply the qualifiers. Some used percentage-based thresholds. Others used normative distributions or clinically defined descriptors. Only one study developed an assessment specifically designed for qualifier use. Explicit differentiation between capacity and performance was rarely reported. Validation of qualifier thresholds was also limited. This means the scores are not reliable enough for broad use.
This doesn't mean this treatment is available yet. The findings highlight a significant gap between theory and practice. The system exists, but it is not being used as intended. This limits the ability to compare data across different settings. It also hinders the development of better interventions. If we cannot measure function accurately, we cannot improve it effectively.
Why Clarity Is Needed Now
The need for clearer definitions is urgent. As the population ages, the demand for precise functional assessments grows. Patients need reliable data to guide their care plans. Insurance companies and policymakers need comparable data to allocate resources. Without standardized criteria, these groups cannot make informed decisions. The current ambiguity leads to inconsistent care.
Experts agree that internationally agreed, category-specific criteria are needed. These criteria would help clinicians apply the qualifiers consistently. They would ensure that a score in one clinic means the same as a score in another. This would enable meaningful, reliable, and comparable assessments of functioning. Until then, clinicians must navigate a system that was not fully operationalized in practice.
What Happens Next
The road ahead requires more research and collaboration. Clinicians must work together to define clear criteria for each category. They need to validate these thresholds across different populations. Future studies should focus on expanding the range of functions assessed. Walking and arm movement are just the start. We need to understand function in daily living tasks.
Approval of new tools will take time. Research takes time to prove safety and efficacy. We cannot expect immediate changes in clinical practice. But the foundation is being laid. Clearer conceptual definitions will guide the next generation of assessments. Patients will benefit from more accurate measurements of their health. This progress will take patience and persistence from the medical community.