The R-factor sorts a liver injury into one of three patterns — hepatocellular, cholestatic or mixed — and it needs a laboratory blood draw: ALT and alkaline phosphatase from the same sample, each read against the upper limit of normal your own lab prints. It is arithmetic on a liver panel: not a home measurement, not a diagnosis.
The ratio was formalized by an international consensus group for Drug-induced liver injury (DILI), where the first question after abnormal enzymes is which structure is hurt: the liver cells, or the bile drainage system. That answer decides what gets investigated next, so it is now used more widely.
What you need
One standard liver panel — the full report, not a summary line.
- ALT (alanine aminotransferase), in the units your report prints.
- ALP (alkaline phosphatase), from the same draw; values weeks apart do not describe one moment.
- The upper limit of normal for each, as printed by that laboratory. Not optional: both limits are assay-specific, the ALT limit usually sex-specific, and the ratio is built out of them.
- Ideally the first abnormal set, taken at the onset rather than weeks into recovery.
- For interpretation, not the calculation: total bilirubin, GGT, AST, and a list of recently started drugs and supplements.
How the measurement works
The two enzymes leak from different places. ALT sits inside hepatocytes and spills into the blood when liver cells are damaged or die. Alkaline phosphatase is concentrated in the cells lining the bile ducts and rises when bile flow is obstructed or those ducts are inflamed.
To compare them, each result is expressed as a multiple of its own upper limit of normal — how many times above the ceiling it sits — which makes the number portable between laboratories. The R-factor is that ALT multiple divided by the ALP multiple. A higher ALT relative to its ceiling pushes R up, toward the hepatocellular pattern; a higher ALP pushes R down, toward the cholestatic one. Enter the two results and their reference limits into the R-factor of liver injury and let it handle the scaling, the step people get wrong by hand.
How to read the result
The output is one number, read against two cut points fixed by expert consensus:
| R value | Pattern | What is being injured |
|---|---|---|
| 5 or above | Hepatocellular | Liver cells — the ALT rise dominates |
| Above 2, below 5 | Mixed | Both, or a pattern in transition |
| 2 or below | Cholestatic | Bile flow and the ducts — the ALP rise dominates |
The pattern is a signpost for the workup. Hepatocellular results usually send a clinician toward viral hepatitis serology, autoimmune markers and recent drugs; cholestatic and mixed results send them toward imaging of the biliary tree. The cut points are conventions rather than biology: 4.9 and 5.1 are not different diseases, and a result near a boundary is ambiguous.
What it does not tell you
It measures neither severity nor cause. Severity is judged from bilirubin, clotting and how the person actually is; a hepatocellular pattern alongside a clearly rising bilirubin is a recognized danger signal that needs a physician the same day. Viral hepatitis, alcohol, a supplement and a gallstone all produce the same ratio. Scarring and chronicity are separate questions, estimated with the FIB-4 Fibrosis Index, elastography or a biopsy.
Both inputs can mislead. Alkaline phosphatase is not liver-specific: bone turnover, a healing fracture, Paget disease and pregnancy raise it, and an ALP coming from bone drags R downward and can fake a cholestatic pattern — which is why a GGT is often added to confirm the enzyme is hepatic. ALT rises after muscle injury and hard exercise, and can look modest in an advanced, burned-out liver. And because the ratio is anchored to reference limits, the same results read at another lab give a different answer.
Timing matters as much as the values. The pattern migrates as an injury evolves, often drifting toward cholestatic during recovery, so a late sample can classify the same episode differently. The ratio was built for suspected drug-induced injury in people who already had abnormal enzymes; applied to one mildly abnormal result in someone well, it is a description, not a finding. And it never decides whether to stop a drug. That is a clinical judgement.
Related reading
- Alkaline phosphatase (ALP)
- Laboratory tests
- Steatohepatosis
- Which Blood Tests Reflect Aging and How to Read Them
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.