Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1078 page)

   One of the more common causes of a false-positive result is unusually rapid clearance of metyrapone from the plasma, resulting in inadequate blockade of cortisol biosynthesis. This is manifested by a serum cortisol concentration >7.5 μg/dL in the sample drawn at 8
AM
in the overnight test, by a serum cortisol concentration >5 μg/dL 4 hours after the last dose of metyrapone, or by urinary cortisol excretion >20 μg/24 hours the day metyrapone was administered in the standard 2-day test.

MICROALBUMIN, URINE

   Definition

   The urine dipstick is a relatively insensitive marker for proteinuria, not becoming positive until protein excretion exceeds 300–500 mg/day. The normal rate of albumin excretion is <20 mg/day (15 μg/minute); persistent albumin excretion between 30 and 300 mg/day (20 and 200 μg/minute) is called microalbuminuria. Albumin excretion >300 mg/day (200 μg/minute) is considered to represent overt or dipstick positive proteinuria (also called macroalbuminuria).

   In type 1 and 2 DM, the presence of microalbuminuria on repeat specimens collected in the basal state may signify early diabetic nephropathy. It is a marker, in patients with or without diabetes, for cardiovascular mortality. For a definition of microalbuminuria, see Table 16.56.

   Measurement of the urine albumin-to-creatinine ratio in an untimed urinary sample is the preferred screening strategy for microalbuminuria. This test has several advantages: it does not require early morning or timed collections, it gives a quantitative result that correlates with the 24-hour urine values over a wide range of protein excretion, it is simple to perform and inexpensive, and repeat values can be easily obtained to ascertain that microalbuminuria, if present, is persistent.

   Other name: albumin/creatinine ratio.

   
Normal range:

   Albumin/creatinine ratio (random urine): <30.0 μg/mg creatinine

   Microalbumin excretion (24-hour urine): 0–29.9 mg/day

TABLE 16–56. American Diabetes Association Definition of Microalbuminuria

   Use

   Diagnosis of kidney dysfunction.