MMSC407 Unit 8: Electrolytes, Osmolality & Trace Elements

Sodium (Na+) reference range

135-145 mmol/L

Sodium (Na+) function

Main extracellular cation and regulates osmotic pressure and fluid balance

Chloride (Cl-) reference range

100-110 mmol/L

Chloride (Cl-) function

Works with sodium to maintain extracellular fluid volume

Bicarbonate (HCO3-) reference range

22-26 mmol/L

Bicarbonate (HCO3-) function

Buffer system component that helps regulate blood pH and osmotic pressure

Potassium (K+) reference range

3.5-5.0 mmol/L

Potassium (K+) function

Nerve and muscle repolarization during action potentials

Calcium (Ca2+) reference range

8.5-10.5 mg/dL

Calcium (Ca2+) function

Muscle contraction and action potential initiation

Magnesium (Mg2+) reference range

1.7-2.3 mg/dL

Magnesium (Mg2+) function

Neuromuscular excitation

Iron (Fe)

Trace element for oxygen transport and storage through hemoglobin and myoglobin

Copper (Cu)

Needed for iron metabolism antioxidant defense and energy production

Zinc (Zn)

Enzyme cofactor involved in immune function and wound healing

Selenium (Se)

Part of antioxidant enzymes

Manganese (Mn)

Enzyme cofactor involved in bone formation

Molybdenum (Mo)

Enzyme cofactor involved in amino acid metabolism

Chromium (Cr)

Enhances insulin action

Cobalt (Co)

Component of vitamin B12

Fluoride (F)

Strengthens tooth enamel

Most common electrolyte analytical method

Ion-selective electrode (ISE)

ISE principle

Ion-specific electrode generates voltage proportional to ion activity/effective concentration

ISE ion-selective membrane

Only permeable to target ion and generates electrochemical potential

ISE internal reference solution

Contains fixed ion concentration for comparison

ISE reference electrode

Provides constant reference voltage

Direct ISE

Measures undiluted sample and preferred for POCT blood gases and avoiding dilution errors

Indirect ISE

Measures diluted serum/plasma and automated analyzers commonly use it (but may falsely decrease electrolytes in hyperlipidemia or hyperproteinemia)

When direct ISE preferred

Hyperlipidemia hyperproteinemia dehydration critical illness suspected pseudohyponatremia POCT rapid monitoring Pseudohyponatremia clue

Low sodium result in lipemic sample →

confirm with direct ISE

Flame photometry principle

Older method measuring electrolyte light emission after flame excitation

Enzymatic method use

Bicarbonate measurement

Colorimetric method use

Calcium and phosphate measurement

AAS (atomic absorption spectrophotometry)

Highly specific method for trace elements and sometimes calcium/magnesium

Gold standard osmolality method

Freezing point depression (cryoscopy)

Freezing point depression principle

More dissolved particles lower freezing point

decrease proportional to osmoles

Osmolality definition

Concentration of osmotically active particles per kg solvent (water)

Serum osmolality reference range

275-295 mOsm/kg

Urine osmolality reference range

300-900 mOsm/kg

Serum osmolality increased causes

Dehydration hypernatremia hyperglycemia alcohol/toxic ingestion

Serum osmolality decreased causes

Overhydration hyponatremia

Urine osmolality increased causes

SIADH dehydration

Urine osmolality decreased causes

Diabetes insipidus AKI

Calculated osmolality formula

2(Na+) + glucose/18 + BUN/2.8

Osmolal gap formula

Measured osmolality - calculated osmolality

Normal osmolal gap

<10 mOsm/kg

Elevated osmolal gap meaning

Suggests unmeasured osmoles

Specimens for electrolyte testing

Serum plasma whole blood urine and selected body fluids

Whole blood use

POCT electrolyte testing with heparinized syringe

Preferred anticoagulant for plasma electrolytes

Lithium heparin

Why avoid EDTA

Binds calcium and magnesium causing falsely low values

Why gently mix heparinized tubes

Prevents hemolysis

Transport/processing requirement

Process within 30 min and transport on ice

Why metal-free tubes needed

Prevent contamination during trace element testing

Hemolysis effect

False increase in intracellular ions released from RBCs

Intracellular ions affected by hemolysis

K+ Mg2+ phosphate

Extracellular ions

Na+ HCO3- Cl- Ca2+

Prolonged tourniquet effect

Blood concentration falsely increases electrolytes

Vigorous tube shaking effect

Can cause hemolysis

Delayed processing issue

pH-sensitive electrolytes especially calcium may change

Hyponatremia causes

SIADH diuretics heart failure vomiting diarrhea renal disease

Hyponatremia symptoms

Headache nausea confusion seizures coma from brain swelling

SIADH definition

Syndrome of inappropriate antidiuretic hormone (excess ADH production)

Hypernatremia causes

Dehydration diabetes insipidus excess sodium intake

Hypernatremia symptoms

Thirst lethargy seizures coma due to brain shrinking

Hypokalemia causes

Diuretics vomiting diarrhea insulin excess alkalosis

Hypokalemia symptoms

Muscle weakness arrhythmias cramps ileus

Hyperkalemia causes

Renal failure acidosis cell lysis ACE inhibitors

Hyperkalemia symptoms

Muscle weakness arrhythmias paralysis

Hypochloremia causes

Vomiting diuretics metabolic alkalosis

Hypochloremia symptoms

Muscle twitching hypotension metabolic alkalosis

Hyperchloremia causes

Dehydration metabolic acidosis excess saline

Hyperchloremia symptoms

Lethargy deep rapid breathing metabolic acidosis

Metabolic acidosis causes

Renal failure diarrhea lactic acidosis ketoacidosis

Metabolic acidosis findings

Low bicarbonate low pH Kussmaul respirations fatigue confusion

Metabolic alkalosis causes

Vomiting diuretics hypokalemia

Metabolic alkalosis findings

High bicarbonate hypoventilation cramps arrhythmias

Hypocalcemia causes

Hypoparathyroidism renal failure vitamin D deficiency

Hypocalcemia findings

Tetany seizures prolonged QT

Hypercalcemia causes

Hyperparathyroidism malignancy vitamin D toxicity

Hypercalcemia findings

Weakness stones abdominal pain short QT

Hypomagnesemia causes

Alcoholism malabsorption diuretics

Hypomagnesemia findings

Tremors arrhythmias muscle spasms

Hypermagnesemia causes

Renal failure excess antacid intake

Hypermagnesemia findings

Hyporeflexia hypotension respiratory depression

Hypophosphatemia causes

Refeeding syndrome alcohol abuse DKA recovery

Hypophosphatemia findings

Weakness bone pain rhabdomyolysis

Hyperphosphatemia causes

Renal failure tumor lysis syndrome

Hyperphosphatemia findings

Soft tissue calcification hypocalcemia-like symptoms tetany

Hyperlactatemia cutoff

>2 mmol/L

Hyperlactatemia causes

Hypoxia sepsis shock mitochondrial dysfunction

Hyperlactatemia significance

Tissue hypoperfusion sepsis severe metabolic stress

Lactate clinical use

Serial measurements track resuscitation success

Anion gap definition

Difference between major measured cations and measured anions

Cation mnemonic

Cats are positive

Anion gap formula

Na+ - (Cl- + HCO3-)

Normal AG without potassium

8-16 mmol/L

AG formula with potassium

(Na+ + K+) - (Cl- + HCO3-)

Normal AG with potassium

10-20 mmol/L