Comparison of visual and turbidimetric methods for determining short-term compatibility of intravenous critical-care drugs.

Visual and turbidimetric methods for determining the short-term compatibility of critical-care i.v. drugs were compared. In phase 1, serial dilutions of calcium chloride and magnesium sulfate were examined visually and turbidimetrically to test the sensitivity of the spectrophotometric method used. In phase 2, i.v. solutions of dobutamine, dopamine, lidocaine, nitroglycerin, and nitroprusside were prepared and studied in all possible combinations of two, three, four and five drugs, for a total of 26 different combinations. In phase 3, 45 two-drug combinations previously reported as physically incompatible were studied. Visual inspection was conducted against a dark and a light background; changes were graded as slight, moderate, or gross. Absorbance was determined at 650 nm; an absorbance value of greater than 0.010 was considered to be evidence of turbidity. Visual, turbidimetric, and pH measurements were done at zero, one, and three hours after mixing. Samples of the calcium chloride-magnesium sulfate mixture that were graded visually as having a slight precipitate had absorbance readings less than 0.010. No physical evidence of incompatibility was observed by either method for dobutamine, dopamine, lidocaine, nitroglycerin, and nitroprusside in any combination. In phase 3, 19 drug combinations were shown to be incompatible; however, only 6 of these (31%) had absorbance readings greater than 0.010 when the visual incompatibility was first observed. There was no physical evidence of incompatibility for several drug combinations that have been listed as incompatible in commonly used references. Turbidimetry does not appear to be as reliable a method for determining the compatibility of drugs as is visual inspection against a dark and a light background.