The caregiver placebo effect in osteoarthritis therapy

Subjective outcome measures, such as lameness scores and owner questionnaires are often used in clinical trials to assess treatments for osteoarthritis. These are useful because they can give an idea of how much a patient’s joint hurts or how much the disease affects the patient’s quality of life. But subjective measures can also be a source of bias in the form of a caregiver (owner or vet) placebo effect, which can make a treatment appear better than it really is.

Recently, researchers set out to measure the placebo effect in caregivers when assessing a treatment for osteoarthritis in dogs.1 They did this by comparing the results of subjective measures of lameness with objective measures. The study included 58 dogs that were in the placebo arm of a larger double-blind, randomised, placebo-controlled trial designed to evaluate the safety and efficacy of an oral NSAID (deracoxib) for the treatment of lameness secondary to osteoarthritis. Both owners and vets were blinded to which dogs were receiving the test drug and which were receiving placebo. The subjective outcome measures (owner questions about lameness and veterinary examination to assess lameness at walk, lameness at trot, and signs of pain on palpation) were compared with the objective outcome measure (ground reaction forces [GRF]). A dog’s gait was considered improved if the GRF increased by at least 5% of its body weight and deteriorated if the GRF decreased by at least 5%. Otherwise, gait was considered unchanged.

When measured objectively over the 42-day evaluation period, limb function improved in 12.1% and worsened in 8.6%, but did not change in most dogs; mean GRF remained unchanged during the trial. By contrast, caregivers (both owners and vets) reported improvements in lameness from the start, with the reported improvements increasing with time. The caregiver placebo effect appeared to be around 57% for owners and 40–45% for vets and was significant (p<0.001) at all assessment time points.

Both objective and subjective outcome measures are needed in clinical trials of osteoarthritis treatment because they measure different aspects of treatment. But it is also important to include a control group in clinical trials to help mitigate the caregiver placebo effect in subjective measures. The study’s authors conclude that vets should also consider placebo effects in interpreting owner responses and veterinary examination findings when assessing treatment effects in clinical practice.

1. Conzemius MG, Evans RB. Caregiver placebo effect for dogs with lameness from osteoarthritis.JAVMA; 2012: 1314–9.

This post was originally published by Veterinary Prescriber in February 2013

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