R-Factor (Drug-Induced Liver Injury Pattern)
R-Factor (Drug-Induced Liver Injury Pattern) - a deterministic tool in Sophie Well's Clinical Scoring & Risk group.
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What this is
Compute the R-factor (Benichou / CIOMS 1990) for suspected drug-induced liver injury: R = (ALT / ALT upper limit) divided by (alkaline phosphatase / ALP upper limit). Sophie Well classifies the result as hepatocellular (R > 5), cholestatic (R < 2), or mixed (R between 2 and 5), which determines the differential and the urgency of stopping the suspect agent.
When to use it
Use this as soon as new liver enzyme elevations show up in a patient on any drug, supplement, or herbal product. The pattern guides workup - hepatocellular injury raises the spectre of acetaminophen, isoniazid, or autoimmune hepatitis; cholestatic injury points at amoxicillin-clavulanate, anabolic steroids, or biliary obstruction. R should be calculated at the time of recognition and re-checked if the pattern shifts; CIOMS is a classification tool, not a causality score (RUCAM does that).
References
Benichou C. CIOMS classification. J Hepatol 1990;11:272-276.
Worked example: Hepatocellular pattern (R = 15).
Sophie Well is a reference and educational tool. Not medical, legal, or financial advice. Does not replace clinician judgment, professional billing review, or legal counsel.