I got it wrong!
This is first correction on Physical Therapy Diagnosis (that I'll admit to) - so don't go telling my wife or my mom!
I posted three days ago that a physical therapist could use a heuristic adjustment (a 'rule-of-thumb') to published estimates of pre-test probabilities for treatment based classification (TBC) groups, for example:
- Stabilization = 33%
- Lumbar Manipulation = 45%
- Thoracic Manipulation for Neck Pain = 54%
The example I used was of my own patient population here on the West Coast of Florida whose demographics I posted here.
My patients were measurably different from published norms for a stabilization group in the TBC derivation study.
I mistakenly recommended an downward adjustment to the pre-test probability based on age. To check my adjustment I e-mailed Dr. Steven McGee, author of Evidence Based Physical Diagnosis, who e-mailed back with the following:
"The only way to adjust published pretest probability is to measure your own clinical experience.
For example, in the last 100 patients you have seen with shoulder pain, how many have had rotator cuff disease?
This is the pretest probability figure to which you would apply Likelihood Ratios of diagnostic tests."
Dr. McGee's book recommends a simple "bedside" approach to using likelihood ratios in the clinic that can assist physical therapists learning about TBC.
Just you and the patient and a few simple tests and measures.
Dr. McGee's Mnemonic
Dr. McGee does a great job in his book but I'll try here to describe the clinical mnemonic:
Memorize 2,5 and 10: these are positive likelihood ratios. The approximate associated upward shifts in post-test probability are +15%, +30% and +45%.
Memorize 1/2, 1/5 and 1/10 (notice the pattern?): these are negative likelihood ratios. The approximate associated downward shifts in post-test probability are -15%, -30% and -45%.
Measuring Treatment Responders for 'Functional' Diagnoses
To measure the pre-test probability of an outcome is the same as a diagnosis. Outcomes what we are trying to improve in physical therapy TBC.
First, test every patient with lower back pain using your stabilization predictor tests:
- Age less than 40
- SLR > 90 degrees
- Positive Prone Instability Test
- Positive Gower's sign
Who is positive on the rule?
For example, The next 100 patients in my clinic who present with lower back pain who meet the stabilization inclusion criteria get tested as described above.
Those with at least 3 positive tests are my treatment responders - let's say 20 patients score at least 3 positive tests.
The pre-test probability is 20/100 = 20%.
At the Bedside
For a patient with 3 or more positive tests the positive likelihood ratio is 4.0. According to Dr. Magee's mnemonic the upward shift is between 15% and 30% (about 25%). Add that to my pre-test probability...
20% + 25% = 45%Stabilization is no better than some randomly chosen clinical intervention for improving this patient's lower back pain. We should try to find an alternative intervention.
By using our own patients'characteristics along with published estimates of likelihood ratios we can "personalize" TBC. As Dr. McGee says in his book...
"...because the best estimate of pretest probability incorporates information from the clinician's own practice - how specific underlying diseases, risks and exposures make diseases more or less likely - the practice of evidence-based medicine is never "cookbook"."Dr. McGee's paper on "Simplifying Likelihood Ratios" is a great way to get started using this valuable tool.
An excellent tutorial on physical therapy diagnosis and the use of pre-test probability is available from the University of Pittsburg and Dr. Julie Fritz.