Table 4.
Caveats and restrictions for tests used to screen for hypercortisolism
| Test | Caveat | When can the test be used despite the caveat? | Verifying the result |
|---|---|---|---|
| All tests | May be falsely normal in a patient with mild or cyclic CS | If the pretest probability is high based on history and physical examination, repeat testing at intervals or when patient feels worst. | Review caveats for each test |
| UFC | May be falsely increased with fluid intake > 4 L/d (106) | If subjects reduce intake | Measure volume; if volume is high, ask about intake |
| May be falsely increased or decreased with incorrect collection | Measure creatinine, which should be ±15% from day to day; check volume | ||
| Will be decreased in the setting of renal impairment (107) | Use with caution when GFR is 30-50 mL/min | Only use result if value is elevated | |
| May be falsely increased with high sodium intake (108,109) | Impact is not clear, as normal ranges may reflect sodium intake; advise patient to reduce sodium when collecting | ||
| May be increased in pseudo-CS (110) | Values > 3-fold upper normal limit are more likely to be true CS | ||
| 1 mg DST | May be falsely normal in Cushing’s disease patients, presumably because of slow drug metabolism (111) | Measure dexamethasone; if in range expected of an 8 mg dose, consider Cushing’s disease | |
| May be falsely abnormal in patients with elevated CBG (2o oral estrogen) or in those with fast metabolism of dexamethasone (112) | Fewest false-positive results in patients not taking medications that interact with CYP3A4 (113) | Measure dexamethasone level—if low, increase dose to achieve correct level; measure CBG—if high, discontinue estrogen for 4-6 weeks | |
| May be falsely abnormal in renal failure (114) | |||
| Bedtime salivary cortisol | May be falsely abnormal in older men and women, and in hypertensive or diabetic patients (115) | If used in these populations, consider accepting only normal results. | |
| May be falsely abnormal in individuals with variable sleeping times (eg, shift workers) (116) | If used in this population, consider accepting only normal results. |
Abbreviations: CBG, corticosteroid-binding globulin; CS, Cushing’s syndrome; 1 mg DST, 1 mg overnight dexamethasone suppression test; salF, salivary cortisol; GFR, glomerular filtration rate; UFC, urinary free cortisol.