Detecting errors in blood-gas measurement by analysis with two instruments.

We performed a two-stage prospective evaluation of the error detection capabilities of duplicate analysis of blood-gas specimens. In the first stage we analyzed 1601 specimens with a Corning Model 175 blood-gas analyzer as the test instrument and a Corning Model 178 analyzer as the reference instrument, and in the second stage we analyzed 1544 specimens with two Model 178 analyzers. In each stage the designated reference instrument underwent troubleshooting whenever an analytical error was detected; the test instrument underwent troubleshooting only when error conditions were indicated by means other than duplicate analysis. An error was considered to have occurred if the difference between the duplicate analyses exceeded 0.02 (for pH), 0.53 kPa, i.e., 4 mmHg (pCO2), or 7% (pO2). The number of specimens for which errors were detected was 97 (6.1%) in the first stage, 23 (1.5%) in the second. For each analyte more errors were detected with the Model 175 analyzer (of older design) than with the newer Model 178 analyzer. Furthermore, in certain periods associated with the use of particular electrodes there were very high error rates for individual analytes: 8% for pCO2, 18% for pO2. We conclude that duplicate analysis should be considered as a possible required standard for error detection.

Resting single-breath diffusing capacity as a screening test for exercise-induced hypoxemia.

Recent reports in selected patients have suggested that a reduced resting single-breath carbon monoxide diffusing capacity may be associated with a fall in arterial oxygen saturation during exercise. To determine if the diffusing capacity could serve as a screening test for changes in oxygen saturation in an unselected population, results of exercise studies were examined in 106 patients consecutively referred to an exercise laboratory. Nearly half of the patients underwent exercise testing to evaluate interstitial disease whereas the remainder were referred for unexplained dyspnea or for disability evaluations. Arterial desaturation was seen within all patient subgroups and was closely associated with reduced diffusing capacity. For detecting changes of 4 percent or more in oxygen saturation, a diffusing capacity of less than 50 percent of predicted gave the best combination of sensitivity (89 percent) and specificity (93 percent), whereas a diffusing capacity of 60 percent or less of predicted was 100 percent sensitive and 64 percent specific. For detecting lesser degrees of desaturation, sensitivities were slightly reduced but specificities were preserved. Thus, a diffusing capacity of less than 50 percent of predicted was associated with substantial arterial desaturation during exercise, whereas patients with a diffusing capacity of more than 60 percent of predicted had no desaturation during exercise. These results suggest that the resting diffusing capacity can serve as a screening test for exercise-induced hypoxemia in an unselected population.