# Assessing Probability

April 1, 2020
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April 8, 2020

There are a number of methods used to assess the probability of an event resulting from medication administration. The Naranjo Scale is perhaps the most common method used. When using this method, the evaluator is prompted to answer each of 10 questions resulting in a final total score. The final assessment is made using the following scale.

Scoring

• ≥ 9 = definite ADR
• 5-8 = probable ADR
• 1-4 = possible ADR
• 0 = doubtful ADR

The FDA developed a modified version of the Naranjo algorithm which includes dietary supplements. However, all methods for evaluating probability are based on the premise of challenge-dechallenge-rechallenge.  The typical presentation for a positive adverse drug reaction usually follows a defined pattern.

First, upon initial challenge with a medication, the patient presents with symptoms, not always consistent with those reported in the literature. If the symptoms are consistent with those previously reported, this increases the probability that the symptoms were secondary to the medication or dietary supplement.

If in the course of treatment, the patient is dechallenged and the symptoms improve or disappear, this increases the probability of causation. The FDA scale is fairly conservative and suggests that after a positive dechallenge, the probability increases to “possible” to “highly possible”.

In unusual circumstances, the patient may be rechallenged. Once this occurs, probability increases if the symptoms reappear. The FDA scale suggests that a positive rechallenge, in conjunction with a confirmed objective test result, are suggestive of a high probability score.

Any of these methods can prove beneficial in assessing probability and causation in a medical malpractice case. There are a number of different methods that can be used. The conclusions are typically very similar but the premise for each is the same; there is a challenge-dechallenge-rechallenge component that allows the evaluator to assess probability.