Overview
Minerals are inorganic elements that are an indispensable part of a complete diet. They are essential for normal growth, maintenance of the body, and the regulation of countless metabolic reactions in the skeleton, tissues, body fluids and digestive juices. Minerals required in an amount greater than 100 mg/day are classified as major elements, while those required in an amount less than 100 mg/day are classified as minor (trace) elements.
This lesson covers the classification, dietary sources, absorption, functions, regulation and clinically important deficiency and toxicity states of the major minerals — calcium, phosphorus and magnesium — and trace minerals — iron, copper, zinc, fluoride, selenium and manganese. As a laboratory professional, recognising abnormal mineral results and correlating them with clinical presentation is a core competency.
Learning Objectives
After this lesson you will be able to…- Classify minerals into major and minor (trace) elements based on daily requirement.
- Describe the absorption, blood transport and functions of calcium and phosphorus.
- Explain the roles of Vitamin D, parathyroid hormone (PTH) and calcitonin in calcium homeostasis.
- Describe the functions, deficiency and toxicity states of magnesium, iron, copper, zinc, fluoride, selenium and manganese.
- Correlate abnormal mineral laboratory results with their clinical significance.
Clinical Story
Why This MattersA 45-year-old woman who underwent thyroid surgery two days ago develops tingling around her mouth, muscle cramps in her hands, and a positive Trousseau's sign. The surgeon orders an urgent serum calcium. The lab technologist must know how accidental parathyroid removal causes hypocalcemia, and must recognise the critical result quickly so the clinician can intervene before laryngeal spasm occurs.
Core Concepts
Total body calcium is 1–1.5 kg, with 99% in bone and 1% in extracellular fluid. The main dietary source is milk, though in India cereals form the major source. Absorption occurs in the 1st and 2nd part of the duodenum via a carrier protein assisted by Ca2+-dependent ATPase. Vitamin D, parathyroid hormone, acidity and amino acids increase absorption, while phytic acid, oxalates, phosphates and malabsorption syndromes decrease it. Normal blood calcium is 9–11 mg/dL.
Calcium functions include muscle excitation-contraction (via calsequestrin), nerve impulse transmission, acting as a second messenger with inositol triphosphate, hormone secretion, reducing capillary permeability, blood coagulation (via prothrombin activation), prolonging cardiac systole, and forming the structural bulk of bone and teeth.
Regulation: Vitamin D (as calcitriol) increases intestinal absorption via calbindin, activates osteoblasts, and increases renal reabsorption. PTH causes bone demineralization and increases renal calcium reabsorption while decreasing phosphate reabsorption. Calcitonin decreases serum calcium by inhibiting osteoclastic bone resorption.
Hypercalcemia (>11 mg/dL) is usually due to parathyroid adenoma or ectopic PTH-secreting tumours, presenting with anorexia, polyuria, confusion, renal stones and osteoporosis. Hypocalcemia (<8 mg/dL) causes tetany, increased neuromuscular irritability, carpopedal spasm and laryngismus, and may be due to accidental parathyroidectomy, autoimmune disease, Vitamin D deficiency or increased calcitonin.
Total body phosphorus is about 1 kg; bone contains 80% and muscle 10%. Daily requirement is 500 mg. Milk (~100 mg/dL) is a good source, with cereals, nuts and meat as moderate sources. Serum phosphate is 3–4 mg/dL in adults and 5–6 mg/dL in children; calcitriol increases its absorption.
Functions include tooth and bone formation, synthesis of high-energy phosphate compounds (ATP, CTP, GTP), nucleotide co-enzymes (NAD, NADP), and the phosphodiester backbone of DNA and RNA. Hypophosphatemia is linked to hypercalcemia and chronic alcoholism; hyperphosphatemia causes cell lysis, hypocalcemia and thyrotoxicosis.
Optimal intake is 300–400 mg/day, sourced from cereals, beans, leafy vegetables and fish. Normal serum level is 1.8–2.2 mg/dL. Magnesium lowers neuromuscular irritability and improves insulin-dependent glucose uptake and tolerance. Hypomagnesemia occurs with liver cirrhosis, protein-calorie malnutrition and hypoparathyroidism. Hypermagnesemia occurs in renal failure, hyperparathyroidism, rickets, oxalate poisoning and multiple myeloma.
Total body iron is 3–5 g; 75% is in blood, the rest in liver, spleen, bone marrow and muscle. Requirement is 20 mg/day (adults), 20–30 mg/day (children), 40 mg/day (pregnancy). Jaggery is the main dietary source; milk is a poor source. Only the ferrous (Fe2+) form is absorbed in the upper duodenum; Vitamin C enhances absorption while phytic and oxalic acid inhibit it.
Absorption is regulated by mucosal, storer and erythropoietic mechanisms — DMT-1 and ferroportin are down-regulated by hepatic hepcidin. Transferrin (a liver glycoprotein) transports iron in blood; ceruloplasmin oxidises Fe2+ to Fe3+. Ferritin is the storage form. Iron deficiency causes microcytic hypochromic anaemia; excess causes hemosiderosis (linked to a gene on chromosome 6).
Total body copper is ~100 mg, found in muscle, liver, bone marrow, brain, kidney, heart and hair. Requirement is 1.5–3 mg/day; only ~10% of dietary copper is absorbed, and excretion is mainly via bile. Copper is required for iron absorption/incorporation into haemoglobin, tyrosinase activity, Vitamin-C-dependent hydroxylation, and raising HDL.
Wilson's disease: a defect in the copper-binding ATPase gene causes reduced ceruloplasmin, hepatocellular degeneration, basal ganglia deposition (lenticular degeneration) and the pathognomonic Kayser–Fleischer ring; treated with a low-copper diet and D-penicillamine. Menke's kidney-hair syndrome is an X-linked defect where copper is absorbed by the gut but cannot be transported into blood.
Daily requirement is ~10 mg. Sources include grains, beans, nuts, cheese, meat and shellfish. Total body zinc is 2 g — 60% in skeletal muscle, 30% in bone. Over 300 enzymes are zinc-dependent (carboxypeptidase, carbonic anhydrase, alkaline phosphatase, lactate dehydrogenase, alcohol dehydrogenase, RNA polymerase). Deficiency causes poor wound healing, skin lesions, impaired spermatogenesis, hyperkeratosis, dermatitis and alopecia. Toxicity (>1000 mg/day) causes gastric ulcer, pancreatitis, anaemia, nausea and vomiting.
Fluoride protects against dental caries at a safe water limit of ~1 PPM; above 2 PPM it causes fluorosis (gastroenteritis, weight loss, osteosclerosis, teeth discolouration), widespread in Punjab, Rajasthan, Delhi and Tamil Nadu.
Selenium requirement is 50–100 µg/day; it is a cofactor for glutathione peroxidase and 5′-deiodinase and is important for sperm maturation. Toxicity (selenosis) causes hair/nail loss, diarrhoea and a garlicky breath odour.
Manganese requirement is 5 mg/day, mainly from nuts; its absorption is inhibited by iron, and it is transported bound to transmanganin and excreted via bile.
Laboratory Principle
Most serum minerals are measured by colorimetric or complexometric reactions: calcium reacts with o-Cresolphthalein Complexone (OCPC) or Arsenazo III to form a coloured complex whose absorbance is proportional to concentration; phosphorus reacts with ammonium molybdate to form a phosphomolybdate complex read colorimetrically; magnesium forms a coloured complex with Calmagite or Xylidyl Blue. Trace minerals such as iron, copper, zinc and manganese can also be quantified by Atomic Absorption Spectrophotometry (AAS), which measures the absorption of light of a characteristic wavelength by ground-state atoms of the element, providing high sensitivity and specificity.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| o-Cresolphthalein Complexone (OCPC) | Working reagent | Serum calcium colour development | 2–8 °C, protect from light |
| Ammonium molybdate reagent | Acidic working reagent | Phosphomolybdate complex for phosphorus | Room temperature, tightly capped |
| Calmagite reagent | Alkaline working reagent | Colour complex for magnesium | 2–8 °C |
| Multi-element calibrator/standard | Traceable to certified reference material | Instrument calibration | 2–8 °C, per manufacturer insert |
Step-by-Step Procedure
Collect venous blood without prolonged tourniquet application into a plain (no anticoagulant) tube; avoid EDTA/citrate/oxalate tubes as they chelate calcium and magnesium.
Allow blood to clot for 20–30 minutes, then centrifuge at 3000–4000 rpm for 10 minutes to separate serum. Avoid haemolysis.
Pipette serum and the appropriate working reagent (e.g. OCPC for calcium) into a cuvette in the ratio specified by the kit insert.
Incubate at the specified temperature (usually room temperature or 37 °C) for the time stated in the kit insert to allow full colour development.
Read absorbance at the specified wavelength against a reagent blank and calibrator, then calculate the concentration and report against the reference range.
Flow Diagram
Quality Control
Run normal and abnormal (two-level) commercial control sera with every batch. Plot results on a Levey-Jennings chart and apply Westgard rules; investigate and correct any run that violates control limits before releasing patient results.
Participate in an External Quality Assessment Scheme (EQAS) for clinical chemistry, comparing your laboratory's mineral results against peer-group consensus values on a regular schedule to detect systematic bias.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Serum calcium > 11 mg/dL | Hypercalcemia — parathyroid adenoma, ectopic PTH tumour | Correlate with PTH, urinary calcium; refer clinically |
| Serum calcium < 8 mg/dL | Hypocalcemia — hypoparathyroidism, Vitamin D deficiency | Urgent clinical alert; check for tetany signs |
| Low serum iron, low ferritin, high TIBC | Iron deficiency anaemia | Check peripheral smear (microcytic hypochromic), stool for occult blood |
| Low ceruloplasmin, Kayser–Fleischer ring | Wilson's disease | Refer for hepatology/neurology work-up |
| Low serum zinc with skin lesions | Zinc deficiency | Dietary assessment; consider supplementation |
Common Errors & How to Avoid Them
Cause: These anticoagulants chelate calcium and magnesium, falsely lowering results.
Prevention: Always use a plain (serum) tube for total calcium and magnesium estimation.
Cause: Red cells are rich in phosphorus, magnesium and potassium; haemolysis falsely elevates these analytes.
Prevention: Use gentle venepuncture technique and avoid vigorous mixing; reject grossly haemolysed samples.
Cause: Causes haemoconcentration and falsely raises protein-bound (total) calcium.
Prevention: Release the tourniquet within one minute of application.
Laboratory Tips from the Bench
Always correct total serum calcium for albumin: for every 1 g/dL decrease in albumin below 4 g/dL, add 0.8 mg/dL to the measured total calcium.
Ionised (free) calcium is the physiologically active fraction and is a better indicator in critically ill patients than total calcium.
Remember "PTH Pulls calcium from bone, Pulls it back in the kidney, but Pushes phosphate out" — PTH raises calcium and lowers phosphate.
Important Notes
Total calcium includes protein-bound, complexed and ionised fractions; only the ionised fraction (~50%) is biologically active. Albumin abnormalities can make total calcium misleading.
Trace mineral samples (zinc, copper, manganese) are highly susceptible to contamination from rubber stoppers, glassware and skin; use trace-element-free collection tubes when these tests are specifically requested.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer.
Clinical Case Study
Apply Your KnowledgeTwo days after total thyroidectomy, the patient develops perioral tingling, cramping in both hands, and a positive Trousseau's sign on examination.
Low calcium with low PTH and reciprocally elevated phosphorus, occurring shortly after thyroid surgery, is characteristic of surgical hypoparathyroidism — likely from accidental removal or devascularisation of parathyroid glands.
- →Hypocalcemia and hyperphosphatemia move reciprocally when PTH is low.
- →Post-thyroidectomy calcium checks are essential to catch this complication early.
- →Critically low calcium results should be flagged and communicated urgently.
Frequently Asked Questions
Red blood cells contain a much higher intracellular concentration of phosphorus, magnesium and potassium than plasma. When cells lyse, these are released into the serum, causing falsely elevated results.
EDTA is a chelating agent that binds calcium and magnesium ions, removing them from solution and making a plain serum tube necessary for accurate total calcium/magnesium estimation.
Major elements are required in amounts greater than 100 mg/day (e.g. calcium, phosphorus, magnesium), while minor or trace elements are required in amounts less than 100 mg/day (e.g. iron, copper, zinc, fluoride, selenium, manganese).
Quick Revision
10-Minute ReviewKey Takeaways
- Minerals are classified as major (>100 mg/day) or minor/trace (<100 mg/day) elements.
- Calcium homeostasis is tightly regulated by Vitamin D, PTH and calcitonin.
- Iron absorption and transport involve DMT-1, ferroportin, hepcidin, transferrin and ferritin.
- Copper disorders — Wilson's disease and Menke's syndrome — arise from defective copper-transporting ATPases.
- Zinc is a cofactor for over 300 enzymes and is essential for wound healing and growth.
- Fluoride, selenium and manganese each have narrow safe ranges between deficiency and toxicity.
Competency Checklist
Track Your MasteryReferences
- Vasudevan DM, Sreekumari S, Vaidyanathan K. Textbook of Biochemistry for Medical Students. 8th ed.
- Harper's Illustrated Biochemistry. 31st ed. McGraw-Hill.
- NIOS Medical Laboratory Technology curriculum — Biochemistry Module, Lesson 11: Minerals.