7.4 Lab Values & Diagnostics Interpretation

Reading a Lab Result Like a Nurse

On the CPNRE and REx-PN, Reduction of Risk Potential (8-14% of the Physiological Integrity category) tests whether you can look at a diagnostic result, recognize a critical value, and choose the correct first action. You rarely order tests on the exam; instead you connect a number to the body system it threatens and to a nursing response. The classic trap is treating a value as trivia rather than asking: what physiological danger does this represent, and what do I do next? Canadian laboratories report in SI units (mmol/L, g/L, umol/L), so memorize ranges in those units rather than the mg/dL values used in some American resources.

Core Normal Ranges (Canadian SI Units)

Ranges vary slightly between labs; the exam expects you to know the direction of danger and the priority action, not decimal precision.

Complete Blood Count (CBC)

Hemoglobin carries oxygen; low values (anemia) produce fatigue, pallor, and compensatory tachycardia, so the nurse monitors oxygen delivery and activity tolerance. White blood cells (WBC) defend against infection. A WBC of 1.5 x10^9/L is severe neutropenia, and the tested priority is to institute protective (neutropenic) precautions: meticulous hand hygiene, no sick visitors, and no unwashed produce or fresh flowers. A high WBC instead suggests infection or inflammation. Platelets below roughly 50 x10^9/L raise bleeding risk, so the nurse holds invasive procedures and prevents injury.

Electrolytes: Sodium, Potassium, Calcium

Potassium has the narrowest safe window of any common electrolyte. A level of 6.5 mmol/L is dangerous hyperkalemia, and the body system at greatest risk is the cardiac system: peaked T waves progressing to lethal dysrhythmias and cardiac arrest. Place the client on a cardiac monitor and notify the prescriber immediately. Potassium is never given as an IV push - it is always diluted and infused slowly by pump. Hypokalemia (below 3.5 mmol/L) also causes dysrhythmias and muscle weakness. Sodium governs fluid balance and neurological status; rapid changes cause confusion or seizures. Calcium controls neuromuscular excitability, so hypocalcemia produces tetany and positive Trousseau and Chvostek signs.

Glucose, HbA1c, and Renal Markers

A capillary glucose below 4.0 mmol/L with symptoms is hypoglycemia: apply the Rule of 15 - give 15 g of fast-acting carbohydrate (about 4 oz of juice or glucose tablets), wait 15 minutes, and recheck. Never give insulin or encourage exercise during a low, because both lower glucose further. HbA1c reflects average glucose over roughly three months, and the general target for many adults with diabetes is below 7.0%. For the kidneys, creatinine is the more specific filtration marker: when both urea (BUN) and creatinine rise together with decreased urine output, kidney filtration and excretion are failing, and the nurse monitors fluid balance, potassium, and uremic signs.

Coagulation, ABGs, Cardiac Markers, Drug Levels

INR monitors warfarin; the therapeutic target for most indications is 2.0-3.0 (antidote vitamin K). The aPTT monitors unfractionated heparin (antidote protamine sulfate). Swapping these two is one of the most common distractors. Read arterial blood gases (ABGs) in three steps - pH, then PaCO2 (respiratory), then HCO3 (metabolic). An example of pH 7.30, PaCO2 55 mmHg, HCO3 24 mmol/L is respiratory acidosis: the low pH is acidotic and the elevated carbon dioxide is driving it, usually from hypoventilation or CO2 retention. Troponin rises with myocardial injury, so a positive troponin plus chest pain is a red flag for an acute coronary event; brain natriuretic peptide (BNP) rises in heart failure. Therapeutic drug levels have a narrow margin: withhold digoxin if the apical pulse is below 60 bpm (count for a full minute), and watch lithium closely because levels above about 1.5 mmol/L are toxic and cause tremor, vomiting, and confusion.

Critical Values and Common Exam Traps

A critical value is a result so far outside normal that it threatens life if not acted upon quickly, and Canadian labs telephone these to the unit. The tested behaviour is a predictable sequence: verify the client and result, assess the client at the bedside, apply the safety intervention (monitor, precautions, hold a drug), and notify the prescriber, then document. Several traps recur. First, do not confuse INR (warfarin) with aPTT (heparin) - a distractor often pairs the wrong drug with the wrong test. Second, on ABGs, always read pH first: if pH and PaCO2 move in opposite directions the cause is respiratory, and if pH and HCO3 move in the same direction the cause is metabolic. Third, hyperkalemia and hypokalemia both endanger the heart, so a potassium abnormality always means cardiac monitoring, never a delayed recheck. A critical value is never something to simply chart and forget - it triggers assessment, monitoring, and prompt communication with the prescriber.