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Using injection of gallium Ga 67 transferrin, technetium Tc 99m red cells, probes over the lungs, and blood samples, a pulmonary leak index (PLI) and pulmonary transcapillary escape rate (PTCER) for transferrin can be measured. This may help differentiating between cardiogenic pulmonary edema (CPE) and permeability (noncardiogenic) pulmonary edema of the acute respiratory distress syndrome (ARDS). The purpose of the study was to evaluate the relative importance of red cell labeling, blood sampling, and probe measurements in this assessment.Analysis of radionuclide data obtained in consecutive patients with radiographic evidence for pulmonary edema, classified as ARDS (n = 13), CPE (n = 8), or mixed (n = 5), was performed. The latter patients met ARDS criteria except for a high pulmonary capillary wedge pressure.The PLI, PTCER, and the 67Ga-lung/blood radioactivity increase (without 99mTc-red cell data) were specific and sensitive indices to differentiate ARDS/mixed from CPE. The blood transcapillary escape rate (TER) of 67Ga-transferrin was about 2- to 6-fold higher in ARDS and mixed than in CPE. The TER had similar diagnostic value as the PLI, PTCER, and the 67Ga-lung/blood radioactivity ratio increase.The diagnostic value of the simple blood TER of 67Ga-transferrin is similar to that of complex methods, using 99mTc-red cells and probe measurements over the lungs, because the complex methods largely depend on the blood TER. Simplification of the method without red cell labeling and probes may facilitate bedside use to diagnose permeability edema of ARDS, particularly in the absence of a pulmonary artery catheter.