DTPA absorption is proportional to liquid absorption after volume addition. The figure compares the radioactive counts of DTPA absorbed to apical liquid volume absorbed at six different time points after the addition of 5 μL of Tc-DTPA in Ringer’s solution (CF HBEs, six cultures included). Liquid volume was measured using a previously described optical technique . Each data point is mean ± SEM. When repeated with different volumes, 2.5 μL: R2 = 0.96, p = 0.003; 10 μL: R2 = 0.56, p = 0.09. When repeated in a different CF cell line, 10 μL: R2 = 0.90, p = 0.004.