Background: The most widely distributed program for computing ejection fraction (EF), end-diastolic volume (EDV) and end-systolic volume (ESV) for both the left ventricle (LV) and right ventricle (RV) from gated blood pool SPECT (GBPS) data is the surface gradient method QBS (Cedars-Sinai Medical Center, Los Angeles, CA). This investigation examined QBS reproducibility and the amount of variation among planar-GBP and QBS LVEF's. Methods: Three technologists independently reconstructed and reoriented GBPS data, from which they independently computed cardiac functional parameters by QBS, and LVEF's from planar-GBP data, for 35 patients (age=59±12 years; 83% male) referred for CAD evaluation (27 with CHF and 25 with prior MI). No technologist altered any automatically-generated QBS outlines for any calculations. Results: All variables were found to be normally distributed (Kolmogorov-Smirnov test). EF, EDV and ESV correlations were significant among all data pairs for the LV (0.98, 0.96 and 0.99) and for the RV (0.74, 0.97 and 0.96) (multiple linear regression). Mean differences were non-significant and 95%-confidence upper limits and lower limits were modest for LVEF's but fairly wide for the other measurements. (Bland-Altman analysis; see Table). Planar LVEF Gated blood pool SPECT LVEF LVEDV LVESV RVEF RVEDV RVESV Upper limit +7% +9% −35 ml −29 ml +14% +34 ml +31 ml Difference 0% −1% −2 ml −2 ml 0% +1 ml −1 ml Lower limit −7% −6% +31 ml +24 ml −14% −31 ml −33 ml Conclusion: QBS ventricular function measurements agree well with those of planar-GBP and are reproducible, with LVEF variance comparable to that of planar-GBP.