Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (701 page)

   False-positive rate equals 2–4%.

   Levels may be normal in lymphoma and lung cancer.

   Serum ACE is significantly reduced in patients on ACE inhibitors (e.g., enalapril and captopril).

   The reference interval for children and adolescents may be as much as 50% higher than specimens from adults.

   Serum ACE abnormality has been reported in 20–30% of α1-antitrypsin variants (MZ, ZZ, and MS Pi types) but in only about 1% of individuals with normal MM Pi type. There is evidence that paraquat poisoning (because of its effect on pulmonary capillary endothelium) is associated with elevated serum ACE.

ANION GAP (AG)

   Definition

   The AG is an arithmetic approximation of difference between routinely measured serum anions (23) and cations (11) = 12 mmol/L.

   Unmeasured ions include proteins (mostly albumin) = 15 mmol/L, organic acids = 5 mmol/L, phosphates = 2 mmol/L, sulfates = 1 mmol/L; total = 23 mmol/L.

   Unmeasured cations include calcium = 5 mmol/L, potassium = 4.5 mmol/L, magnesium = 1.5 mmol/L; total = 11 mmol/L.

   Calculated as Na
⁺
− (Cl
⁻
+ HCO
_3
⁻
);
typical normal values
= 8–16 mmol/L;
if K
⁺
is included, normal
= 10–20 mmol/L; reference interval varies considerably depending on instrumentation and between individuals. Increased AG reflects amount of organic (e.g., lactic acid, ketoacids) and fixed acids present.

   AG initially began as a measure of quality assurance.

   Use

   Identify cause of a metabolic acidosis