Dimensionality and internal consistency of the Jebsen-Taylor Hand Function Test (JTHFT) across affected and non-affected hands: an exploratory cross-sectional study

In the world of rehabilitation, doctors and therapists need reliable ways to measure how well a person can use their hands. They cannot simply ask a patient if they feel better, because feelings are subjective and vary from person to person. Instead, they use performance tests, where a patient is asked to complete specific, everyday tasks while a timer runs. These tests, known as performance outcome measures, provide a direct, objective look at what a hand can actually do, from picking up a coin to turning a page. One of the most famous tools for this is the Jebsen-Taylor Hand Function Test. It asks a person to perform seven different actions, such as writing a sentence, flipping over cards, and moving small objects, to see how long each task takes. For decades, clinicians have trusted the total time it takes to finish all seven tasks as a single, unified score representing the hand's overall ability. However, a lingering question has remained: does this test actually measure one single skill, or is it a mix of different abilities? And does this measurement work the same way for a hand that is injured as it does for a healthy hand?

A team of researchers at the University of São Paulo set out to answer these questions by looking closely at the test itself. They gathered sixty adults who were receiving treatment for injuries or conditions affecting their upper limbs. These participants ranged in age from eighteen to seventy-six and had various diagnoses, including fractures, nerve injuries, and arthritis. Each person performed the full Jebsen-Taylor test with both their injured hand and their healthy hand. The researchers did not just look at the final times; they used statistical methods to see how the seven different tasks fit together. They wanted to know if the tasks were all tapping into the same underlying ability, or if they were measuring distinct things like grip strength, speed, and coordination separately. They also wanted to see if the test was consistent, meaning if the different parts of the test were working together to tell a coherent story about the hand's function.

The investigation revealed a clear picture of how the test behaves. When the researchers analyzed the data, they found that for both the injured hand and the healthy hand, the seven tasks behaved as a single unit. This means the test is unidimensional; it essentially measures one general capacity for manual function rather than a collection of separate skills. However, the reliability of this measurement differed significantly between the two hands. For the injured hand, the test was highly consistent. The seven tasks worked together so well that the total score was a very strong indicator of that hand's ability. But for the healthy hand, the story was different. While the tasks still formed a single group, they did not hang together as tightly. The test was much less consistent when applied to the non-injured hand, suggesting that the total score might not be as trustworthy for a healthy hand as it is for an injured one.

A specific part of the test stood out as the source of this inconsistency: the writing task. In both hands, the task that required writing a sentence was the weakest link. It did not correlate as strongly with the other six tasks as those tasks correlated with each other. When the researchers looked at what happened if they removed the writing task from the calculation, the consistency of the test improved dramatically, especially for the healthy hand. This suggests that writing is a unique skill that depends heavily on factors like education and specific hand dominance, rather than the general manual dexterity measured by the other tasks like moving objects or stacking checkers. The writing task seems to pull the score in a different direction, making the total score less reliable when that task is included.

The researchers also noted an interesting pattern in the timing. For most tasks, the injured hand took longer to complete the work, which is expected. However, for the writing task, the healthy hand actually took longer than the injured hand in this specific group of patients. This counterintuitive result highlights how complex the writing task is; it relies on a lifetime of specialized training that the dominant hand usually possesses. If the injured hand happens to be the dominant one, the patient might struggle with writing, but if the injured hand is the non-dominant one, the dominant hand might still perform the writing task with a level of precision that takes more time to execute than the simpler, more mechanical tasks. This complexity further explains why the writing task disrupts the consistency of the test, particularly when looking at a hand that is not the primary one used for writing.

Ultimately, the study suggests that while the Jebsen-Taylor Hand Function Test is a useful tool, its total score should be interpreted with caution. The test appears to measure a single dimension of hand function, but the consistency of that measurement depends heavily on which hand is being tested and whether the writing task is included. For a patient with an injured hand, the total score is a robust and reliable indicator of their functional capacity. For a healthy hand, the score is less consistent, largely because the writing task introduces a variable that does not fit neatly with the other physical tasks. The researchers conclude that clinicians should be aware of these nuances. They suggest that the writing task might need to be treated differently or excluded in certain contexts to get a clearer picture of a patient's true manual ability, ensuring that the test remains a precise instrument for guiding rehabilitation.