When is an ionised calcium (ionized calcium) test recommended?

When to Order an Ionized Calcium Test

Ionized calcium testing should be performed immediately in patients receiving massive transfusion, those in critical care settings with hemodynamic instability, patients with suspected hypocalcemia presenting with neuromuscular symptoms, and individuals with conditions predisposing to calcium disorders where total calcium measurements may be unreliable. 1, 2

Critical Care and Acute Settings

Massive Transfusion Protocol

• Monitor ionized calcium levels continuously during massive transfusion, as citrate from blood products chelates calcium and predicts mortality better than fibrinogen, acidosis, or platelet counts 1, 2
• Measure ionized calcium at admission in trauma patients, as low levels identify those requiring massive transfusion and correlate with increased mortality 1
• Check levels every 4-6 hours initially until stable, then twice daily during ongoing transfusion 2
• Maintain ionized calcium >0.9 mmol/L minimum (optimal 1.1-1.3 mmol/L) to preserve coagulation and cardiovascular function 1, 2

Septic Shock and Hemodynamic Instability

• Measure ionized calcium in septic shock patients, particularly those with SOFA scores ≥8, as hypocalcemia impairs cardiovascular function and coagulation 2
• Monitor during fluid resuscitation and vasopressor therapy, as even mild hypocalcemia compromises the coagulation cascade 2

Cardiac Surgery and Transplantation

• Perform rapid response ionized calcium testing during cardiac transplantation, as aggressive monitoring prevents cardiac catastrophes and optimizes cardiac function 3
• Test during cardiac surgery when citrate-containing products are administered 3

Symptomatic Hypocalcemia

Neuromuscular Manifestations

• Order ionized calcium immediately when patients present with paresthesias, Chvostek's or Trousseau's signs, bronchospasm, laryngospasm, tetany, or seizures 1, 2
• Test when ionized calcium <0.8 mmol/L is suspected, as this creates significant dysrhythmia risk 2

Cardiac Manifestations

• Measure ionized calcium in patients with unexplained cardiac arrhythmias or QT prolongation 1
• Check levels when cardiovascular dysfunction occurs without clear etiology 2

Specific Patient Populations

22q11.2 Deletion Syndrome

• Perform regular measurements of pH-corrected ionized calcium in all adults with 22q11.2DS, as 80% have lifetime history of hypocalcemia that can recur at any age 1
• Increase monitoring frequency during biological stress: peri-operatively, perinatally, or during severe illness 1
• Test when patients present with fatigue, emotional irritability, abnormal involuntary movements, or seizures 1

Chronic Kidney Disease and Dialysis

• Consider ionized calcium measurement when albumin-corrected calcium classifications seem inconsistent with clinical picture 4, 5
• Use ionized calcium rather than albumin-corrected values in dialysis patients, as albumin correction formulas are poor predictors of true ionized hypo- or hypercalcemia in individual patients 4, 5
• Monitor during dialysis sessions when using low calcium dialysate, as changes in ionized calcium determine blood pressure response and myocardial contractility 6

When Total Calcium is Unreliable

Protein Abnormalities

• Order ionized calcium when serum albumin is abnormal, as total calcium measurements become unreliable 1, 4
• Test when subtle calcium changes are expected or total calcium measurements are inadequate 1
• Measure ionized calcium in critically ill patients where protein binding may be altered 4

pH Disturbances

• Check ionized calcium when significant acid-base disturbances exist, as a 0.1 unit pH increase decreases ionized calcium by approximately 0.05 mmol/L 1
• Measure at the patient's actual pH rather than correcting to standard pH 5

Common Clinical Pitfalls

Laboratory Considerations

• Recognize that standard coagulation tests may appear normal despite significant hypocalcemia-induced coagulopathy, as laboratory samples are citrated then recalcified before analysis 2
• Understand that ionized calcium measurement has worse reproducibility than total calcium and is more expensive, but provides the biologically active fraction 1

Clinical Context Matters

• In hypothermia, hypoperfusion, or hepatic insufficiency, citrate metabolism is impaired and worsens hypocalcemia during transfusion 1, 2
• Colloid infusions (but not crystalloids) independently contribute to hypocalcemia 1, 2

Specific Scenarios Requiring Targeted Testing

• Measure ionized calcium peri-operatively in patients with known calcium disorders 1
• Test during pregnancy and postpartum in at-risk populations 1
• Check levels when patients with hypocalcemia history consume alcohol or carbonated beverages, which can worsen hypocalcemia 1

Normal Reference Ranges

• Normal ionized calcium: 1.1-1.3 mmol/L (4.65-5.28 mg/dL in some references) 1, 2
• Critical low threshold: <0.9 mmol/L requires intervention 1, 2
• Concerning threshold: <0.8 mmol/L significantly increases dysrhythmia risk 2