electrolytes 3( Mg+,Ca+,HPO,lactate)

• 4th most abundant cation in the body (after Na+, K+, Ca2+) • 2nd most abundant intracellular cation (after K+) • Mg2+ in the body can be found in: • Bone (53%) mostly** • Muscle, organs, soft tissue (46%) • Serum and RBCs (<1%) • Magnesium Distribution • of Mg2+ is bound to protein ( mostly albumin ) • 61% exists in the free or ionized state** • 5% complexed with other ions (phosphate, citrate) • Only the free ion is physiologically active** IN A FORM THAT CAN BE USED BY THE BODY

• Cofactor for more than 300 enzymes • Transcellular ion transport • Neuromuscular transmission • Synthesis of carbohydrates, proteins, lipids and nucleic acids • Release of and response to certain hormones

• Raw nuts • Dry cereal • Vegetables • Meats • Fish • Fruit • The small intestine can absorb 20-65% of dietary Mg2+. Regulation

• Regulation of Mg2+ is largely controlled by the kidney. • Reabsorbed when there is a deficit • Excreted when in excess • Reabsorption of Mg2+: • 25-30% of non-protein bound Mg2+ is reabsorbed by the proximal convoluted tubule (less than Na+: 60-75%). • 50-60% is reabsorbed in the ascending Loop of Henle***** • 2-5% reabsorbed in the distal convoluted tubule.

• Parathyroid hormone (PTH) • Increases renal reabsorption of Mg2+ • Enhances absorption of Mg2+ in the intestine • Aldosterone and Thyroxine • Increases renal excretion of Mg2+

• Serum • Lithium heparin plasma • Avoid hemolysis and separate cells from serum ASAP - [Mg2+] inside RBCs are 10X higher than in the ECF • Unacceptable anticoagulants: • Oxalate, citrate, EDTA • They will bind with Mg2+ • Urine -24-hour specimen preferred - Must be acidified with HCl to prevent precipitation

The most common methods involve colorimetry: ** just know that there are 3 main types for colorimetry • Calmagite • Mg2+ combines with calmagite to form a reddish-violet complex • Read @ 532 nm • Formazan Dye • Mg2+ binds with the dye to form a colored complex • Read @ 660 nm • Methylthymol Blue • Mg2+ binds with the chromogen to form a colored complex • Reference method: Atomic Absorption Spectrophotometry (AAS)**

in hospitalized patients in ICUs; rarely seen in non hospitalized patients

Hypomagnesemia causes: reduced intake, decreased absorption, increased excretion symptoms: tetany *** irregular muscle spasms Hypermagnesemia causes: renal failure is most common cause **, increased intake ( renal failure plus taking meds that increase Mg2+) symptoms: GI. decreased thrombin, neurological

0.63-1.0 mmol/L

~ 99% of calcium in the body is part of bone <1% is in the extracellular fluid (ECF) • Calcium exists in plasma in three states: 1) 40% is protein bound - 80% of which is bound to albumin - 20% of which is bound to globulins 2) 15% is bound to anions (HCO3-, citrate, lactate) 3) 45% is ionized (Ca2+) - Physiologically active form - Protein-bound calcium is non-diffusible and cannot pass through capillary walls. - Anion-bound and ionized forms ARE diffusible and can pass through capillary walls.

1) Structural component of bones and teeth 2) Necessary for blood coagulation 3) Required for transmission of nerve pulses } K+ also needed 4) Required for muscle contraction and normal heartbeat } K+ also needed 5) Necessary for activation of some enzymes 6) Role in the transfer of inorganic ions across cell membranes - i.e. membrane permeability

• Maintained by 3 processes:• Absorption (intestine) • Excretion (kidney) • Movement in/out bone

• These processes are regulated by 3 hormones:• Parathyroid Hormone (PTH) • Calcitonin (CT) • Vitamin D • 1,25-dihydroxycholecalciferol (1,25-(OH)2D) is the active form of vitamin D • Converted to the active form by hydroxylation in the kidneys

Increased by: • Increased protein intake (40% Ca bound) • Low gut pH increases solubility• Vitamin D (needed for Ca absorption) • PTH (makes active Vitamin D) Decreased by: • Low protein • High pH decreases solubility • Excess Fatty Acids (complex with Ca) • Phytic acid (insoluble complex) • Increased serum phosphate • Calcitonin • Serum calcium is regulated by: PTH, vitamin D, and calcitonin

• PTH secretion is stimulated by a ↓ in ionized Ca2+ • Secretion is stopped by an ↑ in ionized Ca2+ • Exerts three effects on bone and kidney: In the bone: 1. Activates bone resorption • Activated osteoclasts break down bone and release Ca2+ into ECF In the kidney: • Increases tubular reabsorption of Ca2+ • Stimulates renal production of active vitamin D

• Vitamin D3 is obtained from the diet or exposure of skin to sunlight. • In the liver: • Vitamin D3 is converted to 25-hydroxycholecalciferol (25-OH-D3) • Inactive form of vitamin D • In the kidney: • 25-OH-D3 is hydroxylated to 1,25-dihydroxycholecalciferol (1,25-[OH]2-D3) • Biologically active form • The active form: • ↑ Ca2+ absorption in the intestine • Enhances effect of PTH on bone resorption

• Produced in thyroid gland • Secreted when [Ca2+] in blood ↑ • i.e. secreted in response to a hypercalcemic state • Ca2+ is ↓ by inhibiting the actions of PTH and vitamin D

• Heparinized Plasma or serum collected without venous stasis • (i.e. without a tourniquet or leave tourniquet on for no longer than 1 minute) • Unacceptable anticoagulants:(Will bind with calcium ions and alter the calcium concentration) • EDTA • Oxalate • 24 hour urine • acidified with HCl to keep pH low and prevent calcium precipitation Will bind with calcium ions and alter the calcium concentration

Most common method: • Spectrophotometry of calcium-dye complexes • Orthocresolphthalein complexone (CPC) - Uses 8-hydroxyquinoline to prevent Mg2+ interference • Arsenazo III dye • Reference Method: • Atomic absorption spectrophotometry (AAS) • Rarely used in the clinical setting

• Uses calcium-dye complex (dye binding method) • O-cresolphthalein Complexone method (CPC) - Calcium + o-cresolphthalein ——-> calcium-cresolphthalein complex (purple) - Read at 575nm - The color is proportional to calcium - 8-hydroxyquinoline is used to prevent Mg2+ interference • Vitros uses Arsenazo III dye

• 45% of total serum calcium • Physiologically active form - free calcium ion (Ca2+) • More sensitive marker for calcium disorders

• Heparinized Whole Blood in syringe (recommended) • Easier to keep anaerobic • If not analyzed in 30min, collect on ice slurry • Serum: anaerobic in filled SST vacutainer tube • Keep capped • Clotting & centrifugation should be done quickly (<30 mins) at room temperature • Heparinized plasma and heparinized whole blood in vacutainers can also be used. • Unaffected by slight hemolysis • If sample is grossly hemolyzed - reject - Hemoglobin is released which binds with Ca2+, causing falsely low results

• Increase in pH = ↓ ionized calcium • Proteins will release H+ = more protein with negative charge • Ca2+ will bind with the negatively charged protein → less ionized Calcium • So keep specimen anaerobic • Exposure to air = ↓CO2 = ↑pH = ↓Ca2+ • Decrease in pH = ↑ ionized calcium • Opposite effect → more ionized Calcium