Laboratories are asked to participate in the GH2 proficiency testing (PT) program administered by the College of American Pathologists (CAP). The Survey began in 1996 as a pilot survey and now includes three fresh pooled whole blood specimens (at three GHB/HbA1c levels). Samples are shipped on cold packs using an overnight courier. Target values for each specimen are assigned by the mean result of all SRLs in the U.S. Proficiency testing data are used to assess the effectiveness of the standardization program by: 1) estimation of bias from the target (by laboratory, by method, by method type, and including all GHB methods) and 2) interlaboratory comparability within and between methods. Beginning in 2007, NGSP target values are used for laboratory grading by the CAP.