Communicating calculation of posttest probability using likelihood ratio in one step
I congratulate Steurer et al (ref. 1) for their excellent study showing encouraging results when they presented the positive likelihood ratio in a non-technical language. I have observed while conducting evidence based medicine workshops that the health professionals find the concept of likelihood ratio difficult unless presented in a plain language. They also find it difficult to follow the three steps to determine posttest probability using the pretest probability and likelihood ratio.
Most authors (refs. 2, 3) including Steurer et al (ref 1)first convert pretest probability into pretest odds (step 1), then determine posttest odds by multiplying the pretest odds with the likelihood ratio (step 2) and then convert the posttest odds to the posttest probability (step3). This method requires explaining the concept of odds and its relation with probability, which the health professionals find difficult to follow. As a solution to this, I have derived the following formula, which gives posttest probability (Post-TP)from the pretest probability (Pre-TP) and likelihood ratio (LR)in one step:
Post-TP = (Pre-TP x LR)/[1 + Pre-TP (LR - 1)]
The formula is derived from the formula used in step 2 of the three- step method using simple algebra. This formula does not require use (hence explaining)of the term odds. I understand that Steurer et al(ref 1) did not go for explaining this calculation to the general practitioners. But they indicated that they used the three-step method to do their calculation. It is in this context that I present the above formula. I have found it useful in communicating the calculation to the health professionals. I recommend its use in future write-ups and workshops on evidence-based medicine.
References: 1. Steurer J, Fischer JE, Bachmann LM, Koller M, Rier G. Communicating accuracy of tests to general practitioners: a controlled study. 2. Sackett DL, Haynes RB, Guyatt GH, Tugwell P. Clinical Epidemiology: A Basic Science for Clinical Medicine. 2nd ed. Boston, Mass: Little, Brown & Co; 1991. 3. Jaeschke R, Guyatt GH, Sackett DL for the Evidence-Based Medicine Working Group. Users'guides to the medical literature, III. how to use an article about a diagnostic test, B: What are the results and will they help me in caring for my patients? JAMA 1994;271:703-707.
Competing interests: No competing interests