Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (242 page)

   Limitations

   Causes of false-positive dipstick results

   Delayed processing of hematuria specimen, resulting in hemolysis of RBCs

   Non-Hb urine pigments that may mimic hemoglobinuria (myoglobin, porphyrin)

   Presence of pus, iodides, or bromides

Suggested Readings

Marchand A, Galen RS, Van Lente F. The predictive value of serum haptoglobin in hemolytic disease.
JAMA.
1980;243:1909–1911.

Prahl S. Optical absorption of hemoglobin. http:omlc.ogi.edu/spectra/hemoglobin. December 15, 1999.

HYPEROXALURIAS

   Definitions

   Primary hyperoxalurias (PHs) are rare inborn errors of glyoxylate metabolism, characterized by the overproduction of oxalate, which is deposited as calcium oxalate in various organs, primarily the kidneys. End-stage renal disease results in a significant number of cases. PH types 1–3 stem from autosomal recessive enzymatic defects in

   (PH type 1) the hepatic peroxisomal enzyme alanine:glyoxylate aminotransferase, which is involved in the conversion of glyoxylate to glycine (80% of PH cases)

   (PH type 2) the cytosolic glyoxylate reductase/hydroxypyruvate reductase, which is involved in the conversion of glyoxylate to glycolate (10% of PH cases)

   (PH type 3) the mitochondrial 4-hydroxy-2-oxoglutarate aldolase (5% of PH cases)

   Secondary hyperoxaluria results from increased enteric absorption of oxalate, most commonly caused by fat malabsorption via the binding of calcium by free fatty acids in the colon. This decreases the amount of calcium available to bind to oxalate for the formation of insoluble calcium oxalate, leaving the free oxalate to be more easily absorbed.

   Specific disorders of fat malabsorption result from pancreatic insufficiency, inflammatory bowel disease, bowel resection or jejunoileal or gastric bypass, use of the weight reduction drug orlistat (which causes fat malabsorption by inhibiting gastric and pancreatic lipases), and cystic fibrosis (which causes pancreatic insufficiency and promotes calcium deposition via hypercalciuria).