Evaluation of lymphocyte-predominant pleural effusion
Diagnosis of an exudative pleural effusion that is undiagnosed after cytologic examination (diagnostic in 40–75% of cases)
Recurrent pleural effusion of unknown etiology, pleural mass, or thickening
Interpretation
The test is positive for tumor in approximately 6% of malignant mesotheliomas and approximately 60% of other cases of malignancy.
The test is positive for tubercles in two thirds of cases on first biopsy, with increased yield on second and third biopsies; therefore, repeat biopsy if suspicious clinically. Acid-fast stain or granulomas can be found in 50–80% of cases, and culture of biopsy material for TB is positive in ≤75% of cases. A fluid culture alone establishes a diagnosis of TB in 25% of cases.
POTASSIUM (K)
Definition
Potassium is a primary intracellular ion; <2% is extracellular. High intracellular concentrations are maintained by the Na–K ATPase pump, which continuously transports potassium into the cell against a concentration gradient. This pump is a critical factor in maintaining and adjusting the ionic gradients, on which nerve impulse transmission and contractility of cardiac and skeletal muscle depends. In acidemia, potassium moves out of cells; in alkalemia, potassium moves into cells. Hypokalemia inhibits aldosterone production; hyperkalemia stimulates aldosterone production. Plasma sodium and potassium control potassium reabsorption. Each 1 mmol/L decrease of serum potassium reflects a total deficit of <200–400 mmol; a serum potassium <2 mmol/L may reflect a total deficit >1,000 mmol.
Normal range:
see Table 16.67.
TABLE 16–67. Normal Range for Potassium
Use
Evaluation of electrolyte balance, cardiac arrhythmia, muscular weakness, hepatic encephalopathy, and renal failure
Diagnosis and monitoring hyperkalemia and hypokalemia in various conditions (e.g., treatment of diabetic coma, renal failure, severe fluid and electrolyte loss, effect of certain drugs)
