p16 INK4a (encoded by CDKN2A) and p21 Cip1 (encoded by CDKN1A) are CDK inhibitors that enforce the cell cycle arrest characteristic of senescence. Their expression increases with aging in virtually all tissues and decreases measurably after effective senolytic treatment. In clinical trials, p16 and p21 mRNA levels in skin biopsies or blood cells serve as primary efficacy biomarkers.

Blood p16 mRNA testing is available from specialized longevity labs. Standard commercial labs (LabCorp, Quest) do not currently offer this test. Epigenetic clock tests (TruDiagnostic) may be more practically accessible as a proxy for senescent cell burden.

p16 reference ranges are age-adjusted. The goal of senolytic treatment is to maintain p16 levels at the lower end of your age group's range, or to show a measurable reduction post-treatment cycle.

Does p16 testing tell me specifically how many senescent cells I have?

p16 mRNA from blood reflects the senescent cell fraction in circulating immune cells, which correlates with (but does not precisely quantify) the total body senescent cell burden. Tissue biopsies provide more direct quantification.