Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1296 page)

   After consuming vitamin C, plasma values rapidly rise within 1–2 hours and reach peak concentration within 3–6 hours after ingestion.

VITAMIN D, 1,25-DIHYDROXY

   Definition

   It is the active form of vitamin D and is produced primarily in the kidney by the hydroxylation of 25-hydroxyvitamin D. Other names: calcitriol and 1,25-dihydroxycholecalciferol (1,25-OHD).

   
Normal range:
15–75 pg/mL.

   Use

   As a second-order test in the assessment of vitamin D status, especially in patients with renal disease

   Investigation of some patients with clinical evidence of vitamin D deficiency (e.g., vitamin D–dependent rickets due to hereditary deficiency of renal 1-alpha hydroxylase or end-organ resistance to 1,25-dihydroxyvitamin D)

   Differential diagnosis of hypercalcemia

   Interpretation

Increased In

   Sarcoidosis (synthesized by macrophages within granulomas).

   Non-Hodgkin lymphoma (approximately 15% of cases). Returns to normal after therapy.

Decreased In

   Renal failure

   Hyperphosphatemia