Liver Fat Fraction (MRI-PDFF): What the Percentage Means

Liver fat fraction is a measurement of how much of the liver’s volume is made up of fat, expressed as a percentage. The MRI technique used to measure it is called proton density fat fraction, usually shortened to MRI-PDFF. It works by separating the signal coming from fat molecules from the signal coming from water, then reporting the ratio between them. The scan is quick, requires no contrast dye and no needle, and produces a number that can be compared directly against a scan taken months or years later.

A healthy liver contains a small amount of fat. The threshold most widely used to define hepatic steatosis, meaning excess fat in the liver, is a fat fraction above approximately 5% to 5.6%. Values above that point are generally reported as mild, moderate, or marked steatosis depending on how far above the threshold they sit. This condition is common: roughly a third of American adults meet criteria for what is now called metabolic dysfunction-associated steatotic liver disease, and prevalence rises to well over half among adults with type 2 diabetes or obesity. In 2023, a multi-society consensus renamed the condition from nonalcoholic fatty liver disease, or NAFLD, to MASLD, shifting the definition toward positive metabolic criteria rather than the exclusion of alcohol use. Both terms describe the same underlying process, and our entry on nonalcoholic fatty liver disease covers the condition in more depth.

MRI-PDFF is regarded as the noninvasive reference standard for quantifying liver fat. Standard ultrasound can suggest that a liver looks fatty, but it does so subjectively and performs poorly at lower fat levels. Liver biopsy provides tissue detail that imaging cannot, but it samples a tiny fraction of the organ, carries procedural risk, and is not repeatable on a routine basis. MRI-PDFF measures the entire liver, correlates closely with biopsy findings, and can be repeated as often as clinically useful. That reproducibility is what makes it valuable for tracking change rather than simply confirming a suspicion.

The most encouraging aspect of this measurement is that it responds to what people do. Liver fat is among the most reversible findings in preventive imaging. Weight loss reduces it in a fairly consistent, dose-dependent way, with published data suggesting roughly a 0.77 percentage-point reduction in liver fat for each kilogram of body weight lost. Sustained loss of 7% to 10% of body weight is associated with resolution of liver inflammation and, in some cases, regression of early scarring. Because elevated liver fat is closely tied to insulin resistance and cardiovascular risk, a fat fraction result is often less a diagnosis than a starting point for a broader conversation about metabolic health.

A liver fat fraction result is a number you can act on, and one worth revisiting. To understand more about how this organ works and why it is included in comprehensive imaging, read our article on what the liver is and what it does.