Serum iron; component of iron status.

Hereditary iron overload; iron metabolism.

Insulin; glucose metabolism.

Intrinsic factor; absorption of vitamin B12.

Iodine; thyroid hormone synthesis.

Serum Potassium; cardiac and muscle function.

Bartonella antibodies; for cat scratch disease.

Copper in blood; iron metabolism and enzymes.

Mercury in blood; recent exposure.

Lactate; tissue hypoxia or exertion.

LDH Isoenzymes (1–5); organ pattern.

LDL-cholesterol; cardiovascular risk profile.

Legionella antibodies; suspicion of Legionnaires' disease.

Leptospira‑IgM; recent infection (Weil's disease).

White blood cells; infection or inflammation.

LH as a marker for pituitary function and fertility.

Lipase; in pancreatitis.

Lipoprotein(a); hereditary risk factor for cardiovascular diseases.

Lithium level; therapy monitoring.

Lead in blood; exposure.

Lysine; amino acid status.

Serum magnesium; electrolyte balance.

Intracellular magnesium; additional status assessment.

Malaria diagnostics; antigen or serology.