Nursing care
Critical Lab Values: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
A critical lab value is a result so far outside the normal range that it signals immediate risk to life and requires an immediate provider call. Nurses work from a short memorised list of these thresholds — potassium, glucose, platelets, and others — because recognising the number on sight, without looking it up, is what makes the call fast enough to matter.
What the test measures
Critical lab values are not a single test; they are a designated threshold on common tests — potassium, sodium, glucose, haemoglobin, platelets, white cell count, INR, and several others — that a laboratory flags as immediately life-threatening rather than simply abnormal. Every lab and hospital system sets its own exact cutoffs, published as a critical values list, and nurses are expected to know the values that appear most often on their unit without reaching for the list first.
The point of the test is not diagnosis but urgency. A potassium of 5.3 mEq/L is high but manageable; a potassium of 6.8 mEq/L is a critical value because it can cause a fatal arrhythmia within minutes. The number itself tells the nurse how much time there is to act, which is why speed of recognition matters as much as the number itself.
Normal ranges and what moves them
Normal ranges vary slightly by lab, but common adult reference points include potassium 3.5 to 5.0 mEq/L, sodium 135 to 145 mEq/L, glucose 70 to 100 mg/dL fasting, platelets 150,000 to 400,000/microlitre, and haemoglobin roughly 12 to 17 g/dL depending on sex. Critical thresholds sit well beyond these, for example potassium below 2.5 or above 6.5 mEq/L, or glucose below 40 or above 400 mg/dL.
What moves a value into critical range is usually acute: renal failure driving potassium up, insulin overdose driving glucose down, sepsis consuming platelets, or acute blood loss dropping haemoglobin sharply. Chronic, stable abnormalities rarely trigger a critical flag because the lab's threshold is set to catch sudden, dangerous change, not baseline deviation.
What a high result means
A critically high result usually points to an acute process the body cannot compensate for. High potassium risks cardiac arrest through altered cardiac conduction; high glucose in the critical range suggests diabetic ketoacidosis or hyperosmolar state; a critically high white cell count can indicate overwhelming infection or leukaemia; a critically prolonged INR signals bleeding risk from anticoagulation or liver failure.
In each case the nursing priority is the same: verify the result is not a lab error, assess the patient for corresponding clinical signs, and notify the provider immediately rather than waiting for the next round. A high critical value with no clinical correlation still gets called; it is the lab's job to flag it and the nurse's job to escalate it, not to decide independently that it can wait.
What a low result means
A critically low result often reflects loss, dilution, or failure of production. Low potassium risks the same fatal arrhythmias as high potassium, just through a different mechanism, and often follows diuretic use or gastrointestinal losses. Low glucose below the critical threshold causes seizure and coma within minutes if untreated. Critically low haemoglobin points to acute or ongoing blood loss, and critically low platelets raise the risk of spontaneous bleeding, including intracranial haemorrhage.
Low sodium in the critical range, particularly if it dropped quickly, causes cerebral oedema and seizures, so the correction itself has to be paced carefully rather than rushed. Across all of these, the nurse's first move on seeing the number is to check the patient directly, because a low value that matches a visibly unwell patient is a different emergency than one that appears with a patient sitting up and talking.
Nursing actions by result
Every facility requires a critical value to be reported to the ordering provider within a defined window, often within 30 to 60 minutes, and documented with the time of the result, the time of notification, who was notified, and any orders given. The nurse reads the value back to confirm it was heard correctly, a step that exists because misheard numbers over the phone have caused real harm.
Beyond notification, the nurse assesses the patient for signs consistent with the value — cardiac monitoring for potassium abnormalities, neurological checks for sodium or glucose extremes, bleeding precautions for low platelets or prolonged INR — and prepares for likely orders such as a repeat draw, an ECG, or an immediate treatment like insulin, dextrose, or a potassium-binding agent. Knowing which values trigger which response without hesitating is what keeps the interval between result and treatment short.
Patient preparation and teaching
Before a blood draw, tell the patient what the test checks and whether fasting or timing matters, since some critical values, particularly glucose, are meaningless without knowing when the patient last ate. For patients on medications that shift these values, such as diuretics affecting potassium or insulin affecting glucose, explain the symptoms that should prompt them to seek care between draws, not just wait for the next scheduled test.
After a critical result and its treatment, teach the patient in plain terms what happened and why the response was urgent, since an unexplained sudden intervention is frightening. For patients managing a chronic condition that puts them at recurring risk of a critical value, such as chronic kidney disease and potassium, teach the early warning symptoms so they present before the number reaches a dangerous threshold rather than after.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our dosage calculation and lab values practice questions are the closest set to what this page covers.
Common questions
Do I need to repeat a critical lab value before calling the provider?
Policy varies by facility, but most require the nurse to call immediately on the first result rather than waiting for a repeat draw, since delay can cost the patient time in a genuine emergency. A repeat sample may be drawn afterward to confirm, often at the provider's request.
What if I cannot reach the ordering provider right away?
Escalate through the chain of command specified by policy, typically to a covering provider, charge nurse, or rapid response team, rather than letting the result sit unactioned. Document every attempt and the time of each one.
How do critical values differ between paediatric and adult patients?
Paediatric critical thresholds are often different from adult ones for the same test, particularly for glucose and haemoglobin, because normal physiology varies with age. Always confirm the lab is applying age-appropriate critical ranges rather than adult defaults.
Is a critical value always a true emergency?
Usually, but not always — haemolysed samples, incorrect draw technique, or lab error can produce a falsely critical result. The nurse still reports it immediately and correlates it with the patient's clinical picture rather than dismissing it, since assuming error without checking is the more dangerous mistake.
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