Lab Values & Diagnostics Complete Reference
Chapter 1: How to Think About Lab Values Lab questions are rarely about memorized numbers alone. The exam gives you a value and a patient story, then asks what you do first. The winning habit is to learn each value as a three-part unit: the normal range, the level that becomes dangerous, and the nursing action that follows. A potassium of 5.9 mEq/L is not just a number; it is a patient whose heart rhythm is at risk, whose monitor needs watching, and whose provider needs a call. When you study this way, every value carries its own action, and priority questions become much easier. Three principles organize everything in this book. First, know which direction is dangerous for each value: some values kill when high, like potassium, and some kill when low, like glucose. Second, know which values change slowly and which change fast; sodium shifts cause brain symptoms, and the speed of the shift matters as much as the number. Third, know which results require immediate action and which require monitoring. Critical values are reported promptly to the provider, and the nurse who receives a critical value by phone reads it back to confirm accuracy.
Reading a result in context A single abnormal value means more when you know the patient's baseline, their medications, and their diagnosis. A hemoglobin of 9 g/dL is routine in chronic kidney disease and alarming after surgery with a dropping blood pressure. Digoxin toxicity is judged against the level and the potassium together, because low potassium makes toxicity more likely at a lower digoxin level. Train yourself to ask three questions of every result: what is normal, what changed, and what does this patient take or have that explains it. A nurse receives a critical lab value by telephone. What is the correct action? A. Write it down and chart it at the end of the shift B. Read the value back to the caller to confirm accuracy C. Ask the lab to email the result instead D. Wait for the printed report before acting Answer: B. Critical values are read back to the caller to confirm accuracy, then reported promptly to the provider. Read-back is the standard safety practice for verbal and telephone results.
Chapter 2: Electrolytes: Sodium, Potassium, Calcium, Magnesium Sodium, normally 135 to 145 mEq/L, is the electrolyte of the brain. Hyponatremia causes confusion, lethargy, and seizures, and hypernatremia causes thirst, agitation, and altered consciousness. The speed of correction matters: fixing low sodium too quickly can cause permanent brain injury, so replacement is slow and carefully monitored. Common causes of low sodium include diuretics, excessive water intake, and the syndrome of inappropriate antidiuretic hormone; common causes of high sodium include dehydration and diabetes insipidus. Potassium, normally 3.5 to 5.0 mEq/L, is the electrolyte of the heart. Hypokalemia flattens T waves, causes muscle weakness and leg cramps, and dangerously increases digoxin toxicity risk.
Lab Values & Diagnostics Complete Reference
Hyperkalemia produces peaked T waves, widened complexes, weakness, and can progress to fatal arrhythmias. Intravenous potassium is always diluted and infused slowly by pump, never pushed, because a rapid push can stop the heart. A patient with a potassium of 6.0 or higher needs cardiac monitoring and urgent treatment, typically with insulin and glucose, and calcium gluconate to protect the heart membrane. Calcium, normally 8.5 to 10.5 mg/dL, governs nerves, muscles, and the heart. Low calcium causes tingling around the mouth, positive Chvostek and Trousseau signs, and tetany, classically after thyroid or parathyroid surgery. High calcium causes lethargy, constipation, and shortened heart intervals, often from malignancy or hyperparathyroidism. Magnesium, normally 1.3 to 2.1 mEq/L, travels with calcium and potassium; low magnesium makes low potassium impossible to correct, and high magnesium, as from excessive antacid use or magnesium sulfate therapy, depresses reflexes and respirations. Value
Normal; Danger direction; Key nursing action; Sodium; 135-145 mEq/L; Both, brain symptoms; Seizure precautions, slow correction; Potassium; 3.5-5.0 mEq/L; High is lethal; Cardiac monitor, never IV push; Calcium; 8.5-10.5 mg/dL; Low, tetany; Check Chvostek/Trousseau, airway; Magnesium; 1.3-2.1 mEq/L; High, depressed reflexes; Check deep tendon reflexes
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