I131-Tagged Bilirubin Tolerance Test: Preliminary Report

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Several radioisotopic liver-function tests have been described in recent years, but none has gained widespread clinical acceptance. Taplin (7, 5) developed the radioactive rose bengal uptake-excretion test, and McLaren (3) utilized radioactive iodipamide. A new agent, I131-tagged bilirubin,2 has been used in what is essentially an adaptation of the stable bilirubin tolerance test, which proved to be impractical for technical reasons (1, 6). The new technic combines the advantages of a tolerance test, which measures the dynamic response of the liver, with the use of bilirubin instead of a substance foreign to the body. Bilirubin, in its physiological form, is unconjugated and bound to serum protein (1, 4). This complex is not excreted in the urine and is transported to the liver. Bilirubin diglucuronide is conjugated and excreted into the small intestine (1). I131-tagged-unconjugated bilirubin bound to serum albumin as used in this study was prepared by iodination of bilirubin with I131 on a mole for mole basis. After iodination the monoiodo derivative was stabilized with phosphate buffer (pH 7.6) and sodium ascorbate (100 mg./ml.) ; then mixed with human serum albumin in the proportion of 20 mg. of bilirubin for each gram of albumin. After one half hour at 5° C, it was dialyzed with phosphate buffer overnight to remove free iodide. This complex was assayed chromatographically, with the phosphate buffer-sodium ascorbate solution as the solvent, and shown to be stable under the experimental conditions. Martin (2) has shown by spectrophotometric and electrophoretic methods that serum albumin forms a complex with bilirubin, on addition, that is stable over the physiological pH range. Control serum bilirubin determinations were made on hospital patients with and without evidence of liver disease. Preparation consisted in an overnight fast and oral administration of 3 drops of Lugol's solution. Ten microcuries of I131-bilirubin-albumin complex was injected intravenously. Blood radioactivity levels were determined in a well scintillation counter at five minutes, twenty minutes, one hour, and three hours. Twenty-four-hour urine specimens were collected and radioactivity percentage levels were determined (see accompanying table). There appears to be a correlation inversely of serum bilirubin with twenty-four-hour urine percentage recovery. Jaundiced patients showed a range from 20 to 40 per cent, while the normal range averaged 50 to 70 per cent. The one exception is patient No. 8 with infectious hepatitis, in the second month of illness. Blood radioactivity levels showed no correlation with the clinical picture. Continuous measurement with a scintillation probe over the thigh and an Esterline-Angus recorder showed a rapid rise and then constant activity levels over the next three hours. Conclusions These findings suggest that I131 is separated from bilirubin by liver cells in proportion to the state of liver function.