title: What is the true number needed to screen and treat to save a life with prostate-specific antigen testing?
abstract: OBJECTIVE
  The European Randomized Study of Screening for Prostate Cancer (ERSPC) reported a 20% mortality reduction with prostate-specific antigen (PSA) screening. However, they estimated a number needed to screen (NNS) of 1,410 and a number needed to treat (NNT) of 48 to prevent one prostate cancer death at 9 years. Although NNS and NNT are useful statistics to assess the benefits and harms of an intervention, in a survival study setting such as the ERSPC, NNS and NNT are time specific, and reporting values at one time point may lead to misinterpretation of results. Our objective was to re-examine the effect of varying follow-up times on NNS and NNT using data extrapolated from the ERSPC report.
  
  
  METHODS
  On the basis of published ERSPC data, we modeled the cumulative hazard function using a piecewise exponential model, assuming a constant hazard of 0.0002 for the screening and control groups for years 1 to 7 of the trial and different constant rates of 0.00062 and 0.00102 for the screening and control groups, respectively, for years 8 to 12. Annualized cancer detection and drop-out rates were also approximated based on the observed number of individuals at risk in published ERSPC data.
  
  
  RESULTS
  According to our model, the NNS and NNT at 9 years were 1,254 and 43, respectively. Subsequently, NNS decreased from 837 at year 10 to 503 at year 12, and NNT decreased from 29 to 18.
  
  
  CONCLUSIONS
  Despite the seemingly simplistic nature of estimating NNT, there is widespread misunderstanding of its pitfalls. With additional follow-up in the ERSPC, if the mortality difference continues to grow, the NNT to save a life with PSA screening will decrease.
authors.affiliation: Brady Urological Institute, Johns Hopkins Medical Institutions, Baltimore, MD, USA.
authors.forename: Stacy
authors.initials: S
authors.lastname: Loeb
authors.forename: Edward F
authors.initials: EF
authors.lastname: Vonesh
authors.forename: E Jeffrey
authors.initials: EJ
authors.lastname: Metter
authors.forename: H Ballentine
authors.initials: HB
authors.lastname: Carter
authors.forename: Peter H
authors.initials: PH
authors.lastname: Gann
authors.forename: William J
authors.initials: WJ
authors.lastname: Catalona
mesh_terms: D003627:Data Interpretation, Statistical; D005060:Europe / Q000453:epidemiology; D006801:Humans; D015993:Life Tables; D008297:Male; D008403:Mass Screening / Q000706:statistics & numerical data; D011237:Predictive Value of Tests; D011379:Prognosis; D017430:Prostate-Specific Antigen / Q000032:analysis*; D011471:Prostatic Neoplasms / Q000175:diagnosis* / Q000401:mortality* / Q000517:prevention & control; D016032:Randomized Controlled Trials as Topic; D018570:Risk Assessment; D012307:Risk Factors; D018401:Sample Size; D016019:Survival Analysis; D015996:Survival Rate; D013997:Time Factors
chemical_list: D017430:Prostate-Specific Antigen