Lab Tests Reference Library

25-hydroxy Vitamin D (calcidiol) is the storage form of vitamin D and the best test to measure overall vitamin D status. It reflects vitamin D obtained from sunlight exposure, diet, and supplements. Vitamin D is essential for calcium absorption, bone mineralization, immune function, and muscle strength.

Vitamin B12 (cobalamin) is a water-soluble vitamin essential for DNA synthesis, nerve myelin sheath maintenance, and red blood cell formation. It is found only in animal products, making strict vegans at high risk of deficiency. B12 absorption requires intrinsic factor — a protein produced by stomach parietal cells — making gastric disorders another major risk factor.

Folate (vitamin B9) is a water-soluble B vitamin essential for DNA synthesis, cell division, and amino acid metabolism. It is especially critical during rapid cell division in embryonic development — folate deficiency in early pregnancy is a leading cause of neural tube defects (spina bifida, anencephaly). Folate is found in leafy green vegetables, legumes, fortified cereals, and liver.

Ferritin is the main iron storage protein, found primarily inside cells of the liver, spleen, and bone marrow. A small amount circulates in blood and is the most sensitive serum marker of total body iron stores. Ferritin is also an acute-phase reactant — it rises in inflammation, masking iron deficiency when both conditions coexist.

Serum iron measures the amount of iron circulating in the bloodstream bound to transferrin. It fluctuates significantly throughout the day (diurnal variation) and is affected by recent iron intake, making it less reliable as a standalone test. Serum iron is most useful when interpreted alongside TIBC and ferritin to assess iron status comprehensively.

TIBC measures the maximum amount of iron that blood can carry — it reflects the total amount of transferrin available to bind iron. Transferrin is produced by the liver and is the transport protein for iron. In iron deficiency, the liver produces more transferrin (high TIBC); in iron overload or chronic disease, the liver produces less (low TIBC).

Calcium is the most abundant mineral in the body, with 99% stored in bones and teeth and 1% in blood and soft tissues. In blood, calcium exists in three forms: ionized (free, 50% — biologically active), protein-bound (mainly albumin, 40%), and complexed (10%). Total serum calcium measures all three fractions and must be corrected for albumin levels.

Magnesium is the second most abundant intracellular cation and a cofactor in over 300 enzymatic reactions, including energy production (ATP synthesis), protein synthesis, DNA replication, and membrane transport. Approximately 60% is stored in bone, 38% in cells, and only ~2% in blood. Because most magnesium is intracellular, serum magnesium can be normal even with significant total body depletion.

Zinc is an essential trace mineral that serves as a structural component of hundreds of enzymes and transcription factors involved in immune function, wound healing, DNA synthesis, cell division, and sensory function (taste and smell). The body has no zinc storage system — a regular dietary supply is essential. Zinc deficiency is the fifth leading nutritional risk factor for disease globally.