lab timing
2 interactions related to lab timing
iron + ferritin test
Recent iron supplementation does not meaningfully affect ferritin measurement itself (ferritin is a storage protein, not free iron in serum), but it does spike serum iron and transferrin saturation values measured in the same panel, leading to misleading interpretation of overall iron status. A ferritin result drawn shortly after an iron tablet can also be misinterpreted alongside falsely elevated serum iron, suggesting iron stores are repleted when they are not.
vitamin d + parathyroid hormone test
Vitamin D supplementation does not chemically interfere with the parathyroid hormone (PTH) assay itself, but it physiologically suppresses PTH secretion by raising serum 25-hydroxyvitamin D and calcium levels. A PTH drawn after starting or adjusting vitamin D can therefore look lower than the patient's true baseline, complicating workup of suspected primary hyperparathyroidism or vitamin D deficiency.