The P4 pain scale compares favorably with other commonly used measures for patients with severe knee osteoarthritis (OA), a Canadian study has found.
When compared against the Western Ontario and McMaster Universities Arthritis Index pain subscale (WOMAC-pain) and the Intermittent and Constant Osteoarthritis Pain (ICOAP) scale, P4 “had comparable reliability, internal consistency, and responsiveness to OA disease specific pain measures”, the researchers report.
They performed a secondary analysis of data from a randomized controlled trial of 156 participants with knee OA who were consulting a surgeon regarding knee arthroplasty, asking patients to complete the pain measures – P4, WOMAC-pain and ICOAP, as well as the function subscale of WOMAC (WOMAC-function) – at baseline.
Participants were then randomly assigned in a 1:2 ratio to a control or intervention group, with the latter prescribed one of two courses of exercise, and after 2 weeks they were retested on all scales.
All participants had a radiologically confirmed diagnosis of knee OA, and had experienced pain for at least 4 months.
The P4 pain scale, which measures pain intensity through four items on an 11-point numeric scale from 0 (no pain) to 10 (pain as bad as it can be), was found to have low floor and ceiling effects (<1%). Its test–retest reliability, internal consistency and responsiveness were similar to the values for WOMAC-pain and ICOAP.
Importantly, the factorial structures of P4 and ICOAP were separate from WOMAC-function items, indicating that they measure different things. By contrast, WOMAC-pain and WOMAC-function subscale items loaded on similar factors, suggesting overlap.
The ICOAP, which has two different subscales measuring constant and intermittent pain, respectively, comprises 11 items using a 5-point scale from 0 to 4. The researchers found that the ICOAP-constant subscale had a large floor effect, with 33% of patients having the minimum score. They postulate that this could be due to the participants tending to need clarification of what constant pain meant. Perhaps their clarification of “pain all the time” led to the scale being misinterpreted, they comment.
However, given that the floor effect for the ICOAP total score was only 1%, they point out that “[E]ven if patients do not have constant pain, their pain status can still be captured utilizing ICOAP-intermittent subscale and total score.”
Writing in Osteoarthritis and Cartilage, lead author Shawn Robbins (McGill University, Montreal) and colleagues go on to say that P4 is a generic pain scale designed for use in a variety of musculoskeletal conditions, and that it is quick and easy to complete.
Consequently, although they caution that it should be tested in populations with less severe OA, they conclude that “P4 is a generic pain measure that can be utilized to measure pain in patients with knee OA.”
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