Results of a Prospective Study Evaluating a Noninvasive Method of Hemoglobin Adjustment for Determining the Diffusing Capacity of the Lung

Rationale: Measurement of the diffusing capacity of the lung for carbon monoxide (Dl_CO) is significantly influenced by the pulmonary capillary blood volume. Consequently, measurements require adjustment for blood hemoglobin concentration (Dl_COadj) to allow meaningful clinical interpretation. Noninvasive point-of-care devices that measure hemoglobin transcutaneously provide immediate values for hemoglobin that may be useful in pulmonary function laboratories for determining Dl_COadj.

Objectives: To test the hypothesis that Dl_COadj determinations obtained with a commercially available device for noninvasive, point-of-care measurement of blood hemoglobin concentrations are not significantly different from determinations obtained using hemoglobin concentrations measured conventionally in venous blood samples.

Methods: In a prospective open trial, hemoglobin measurements were obtained with the Pronto-7 spot check pulse CO-oximeter (Massimo, Irvine, CA) and by venipuncture for 205 patients referred for Dl_CO testing at Cincinnati Children's Hospital. Hemoglobin and Dl_COadj measurements were compared between the two methods, using Student paired t tests and Bland-Altman plots. To assess variability, the differences in Dl_COadj between the two methods were also compared by a modification of the current standard for acceptable within-session variability for Dl_CO. Clinical interpretation for individual Dl_CO tests based on Dl_COadj values obtained from the two methods were compared statistically using Kendall's coefficient of concordance to determine whether the Pronto-7 altered the classification of the severity of Dl_CO defects.

Measurements and main results: Measurements of hemoglobin concentration by the Pronto-7 analyzer were significantly lower than those obtained from venipuncture blood samples (13.1 ± 1.8 vs. 13.4 ± 2.0; P = 0.01). However, there were no differences for Dl_COadj between both methods (23.6 ± 7.7 vs. 23.7 ± 7.5 ml/min/mm Hg; P = 0.42). There was strong correlation between the Pronto-7 and venipuncture Dl_COadj values (r = 0.99, P < 0.0001). Variability between the two methods was low for Dl_COadj, with a bias of -0.07. More than 96% of tests met acceptable within-session variability. There was no significant difference in the clinical interpretation of the Dl_CO test based on Dl_COadj values recovered from both methods (Kendall's coefficient, 0.96).

Conclusions: Noninvasive measurement of hemoglobin for determination of Dl_COadj was accurate and provided acceptable within-session variability. The results obtained noninvasively did not alter clinical interpretation of test results compared with venipuncture. These findings support noninvasive point-of-care devices as an alternative to venipuncture for determining hemoglobin to measure Dl_COadj in most patients.