Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1188 page)

Decreased In

   False-negative results if patient is edematous or if an inadequate quantity of sweat is collected and analyzed

   Methodologic and technical errors

   Limitations

   Values may be increased to CF range in healthy persons when sweat rate is rapid (e.g., exercise, high temperature), but pilocarpine test does not increase sweating rate.

   Mineralocorticoids decrease sodium concentration in sweat by approximately 50% in normal subjects and 10–20% in CF patients whose final sodium concentration remains abnormally high.

   Confirmation of a diagnosis of CF requires two positive sweat tests done on different days. Borderline results should be reported with the suggestion that the test be repeated if clinically indicated.

   The preferred patient age for testing is after 48 hours. During the first 24 hours after birth, sweat electrolytes are transiently elevated and rapidly decline on the 2nd day. Therefore, sweat testing should not be performed within 48 hours after birth.

Suggested Readings

Boat TF, Acton JD. Cystic fibrosis. In: Kliegman RM, Behrman RE, Jenson HB, et al. (eds.).
Nelson Textbook of Pediatrics
, 18th ed. Philadelphia, PA: Saunders Elsevier; 2007:1803–1817.

Farrell PM, Rosenstein BJ, White TB, et al. Guidelines for diagnosis of cystic fibrosis in newborns through older adults: cystic fibrosis consensus report.
J Pediatr.
2008;153(2):S4–S14.

RED BLOOD CELLS (RBCs): COUNT AND MORPHOLOGY
*

   Definition and Use

   The RBC count is part of the CBC as obtained by automated counters. It is less useful than the Hb or Hct.

   
Normal range:
4.2–5.4 cells/μL in women and 4.4–6.0 cells/μL in men (reported by automated counters in a random adult population)

   Different values are reported for newborns, infants, and children until they reach adulthood.

   Automated counters adjust normal values for age groups.