Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (741 page)

   Chronic diarrhea

   Salicylate overdose

   Limitations

   Bicarbonate can be determined by titration, but this is rarely done.

   HCO
₃
− is the largest fraction contributing to the total CO
₂
. Therefore, both parameters usually change in the same direction.

   The standard HCO
₃
− is the concentration of HCO
₃
− in whole blood at 38°C equilibrated at a PCO
₂
of 40 mm Hg with the blood Hb fully oxygenated.

BILIRUBIN; TOTAL, DIRECT, AND INDIRECT

   Definition

   These assays are commonly used tests to assess liver function. Daily production of unconjugated bilirubin is mainly from senescent erythrocytes. The half-life of unconjugated bilirubin is <5 minutes. UDP-glucuronyl transferase catalyzes rapid conjugation of bilirubin to the liver; conjugated bilirubin is excreted in bile and is essentially absent from the blood of the normal individuals. Delta bilirubin (bili protein) is produced by reaction of conjugated bilirubin with albumin, and its half-life is 17–20 days. Bilirubin is typically measured in two assays for “total” and “direct”; subtracting direct from total gives “indirect bilirubin.” The direct bilirubin measures the majority of delta and conjugated bilirubin and a small percentage of unconjugated bilirubin.

   
Normal range:
age dependent (see Table 16.12).

TABLE 16–12. Normal Range of Bilirubin

   Use

   Assessing liver function

   Evaluating a wide range of diseases affecting the production, uptake, storage, metabolism, or excretion of bilirubin