Sunday, July 28, 2013

minimal clinically important difference (MCID) vs I/MPI

Investigating minimal clinically important difference for Constant score in patients undergoing rotator cuff surgery

There has been a recent fascination with the minimal clinically important difference as a unit of treatment effectiveness. These authors prospectively analyzed collected cohort of 802 consecutive shoulders with arthroscopically treated partial- or full-thickness rotator cuff tears.. The Constant score was measured preoperatively and at 3 months and 1 year postoperatively. At follow-up visits, the patients were asked a simple 2-stage question: Is the shoulder better or worse after the operation compared with the preoperative state? This single 2-level question was used as an indicator of patient satisfaction and as an anchor to calculate the MCID for the Constant score.

The preoperative Constant score averaged 53.1 (SD 17.2) in all patients, 56.2 (SD 17.4) in male patients, and 48.2 (SD 15.6) in female patients. Postoperatively at 3 months, the scores were 61.7 (SD 16.4) in all patients, 65.1 (SD 16.1) in male patients, and 56.8 (SD 15.5) in female patients.  At 3 months postoperatively, 92.2% of male patients and 87.2% of female patients were satisfied with the outcome (P= .027). The 3-month mean change estimate of MCID was 10.4 points.

Comment: As surgeons, our goal is to restore as much of the patient's lost function and comfort as possible. The problem with the MCID is that an improvement of 10 points from a Constant score of 10 to 20 is not the same as an improvement of 10 points from a Constant score of 60 to 70, in that it does not reflect the amount of improvement in relation to the improvement possible.  As an alternative, we have used the I/MPI or percent of possible improvement as metric for measuring the result of treatment. The I/MPI is easily calculated as:

(score at the time of follow-up - score before surgery) 
divided by 
(perfect score - score before surgery) 

The beauty of the I/MPI  is that it can be used for any scoring system (Constant, SST, WOMAC, UCLA,  ASES, etc). Thus, taking the example above, if a perfect score is 100 and the patient records 60 before and 70 after surgery (improving by the MCID), the I/MPI  is

(70 - 60)/(100 - 60) 
25% (the patient improved 25% of the possible improvement).

On the other hand, if the patient records 10 before and 20 after surgery (again improving by the MCID), the I/MPI  is

(20-10)/(100 - 10) 
11% (the patient improved 11% of the possible improvement).

The I/MPI  (percentage of possible improvement) has the following advantages:
(1) It measures what is most important to the patient (how much better is the shoulder after treatment).
(2) It enables comparison of studies done using different outcome metrics (i.e. 'normal' is converted to 100%)
(3) It avoids the problem of having different MCIDs for each condition (arthritis, instability, cuff disease), for different genders, and for different instruments (ASES, SST, Constant). 

Try it, you will like it! 
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