Errors, mistakes, blunders, outliers, or unacceptable results: how many?

We have studied 219353 individual clinical chemistry results obtained in methods comparison studies. Each result was prospectively compared with its replicate, comparative, or repeat value to identify differences from expected values. Unacceptable results were defined as differing from the expected values by < or = 7 SDs or CVs. We believe these differences represent special-cause variation and should be expressed as unacceptable rates per million results (ppm). We observed 447 ppm unacceptables: 196 ppm in control samples and 251 ppm in patients' samples. Results judged likely to alter patient care occurred at a rate of 41 ppm. To better understand the magnitude of these rates, we compared these results with reports of error rates in HIV testing and the airline industry. The measurements reported were made for the purpose of quality improvement, not judgment or discovery. The significance of these findings for laboratorians, manufacturers, and regulators is discussed.