BACKGROUND: Hutchinson-Gilford progeria syndrome (HGPS) is an ultrarare, fatal, premature aging disease caused by a toxic protein called progerin. Circulating progerin has not been previously detected, precluding research using readily available biological samples. This study aimed to develop a plasma progerin assay to evaluate progerin's quantity, response to progerin-targeted therapy, and relationship to patient survival. METHODS: Biological samples were collected by The Progeria Research Foundation Cell and Tissue Bank from a non-HGPS cohort cross-sectionally and a HGPS cohort longitudinally. HGPS donations occurred at baseline and intermittently while treated with farnesylation inhibitors lonafarnib±pravastatin and zoledronate, within 3 sequential open-label clinical trials at Boston Children's Hospital totaling >10 years of treatment. An ultrasensitive single-molecule counting progerin immunoassay was developed with prespecified performance parameters. Intra- and interpatient group statistics were descriptive. The relationship between progerin and survival was assessed by using joint modeling with time-dependent slopes parameterization. RESULTS: The assay's dynamic detection range was 59 to 30 000 pg/mL (R2=0.9987). There was no lamin A cross-reactivity. Mean plasma progerin in non-HGPS participants (n=69; 39 male, 30 female; age, 0.2-71.3 years) was 351±251 pg/mL, and in drug-naive participants with HGPS (n=74; 37 female, 37 male; age, 2.1-17.5 years) was 33 261±12 346 pg/mL, reflecting a 95-fold increase in affected children (P<0.0001). Progerin levels did not differ by sex (P=0.99). Lonafarnib treatment resulted in an average per-visit progerin decrease from baseline of between 35% to 62% (all P<0.005); effects were not augmented by adding pravastatin and zoledronate. Progerin levels fell within 4 months of therapy and remained lower for up to 10 years. The magnitude of progerin decrease positively associated with patient survival (P<0.0001; ie, 15 000 pg/mL decrease yields a 63.9% decreased risk of death). For any given decrease in progerin, life expectancy incrementally increased with longer treatment duration. CONCLUSIONS: A sensitive, quantitative immunoassay for progerin was developed and used to demonstrate high progerin levels in HGPS plasma that decreased with lonafarnib therapy. The extent of improved survival was associated with both the magnitude of progerin decrease and duration at lower levels. Thus, plasma progerin is a biomarker for HGPS whose reduction enables short- and long-term assessment of progerin-targeted treatment efficacy. REGISTRATION: URL: https://www. CLINICALTRIALS: gov. Unique identifiers: NCT00879034 and NCT00916747.