11.6 Laboratory Testing in Cardiac Care
Key Takeaways
- High-sensitivity cardiac troponin is reported in ng/L and uses sex-specific 99th-percentile upper reference limits (women lower than men); acute myocardial injury requires a rising or falling pattern with at least one value above the upper reference limit, while an elevated but flat value is chronic injury.
- An elevated troponin means myocardial injury, not necessarily type 1 myocardial infarction: demand ischemia, myocarditis, heart failure, tachyarrhythmia, pulmonary embolism, sepsis, chronic kidney disease, cardiac contusion and post-cardiac-surgery states all raise it.
- NT-proBNP below 300 pg/mL rules out acute heart failure at any age; rule-in thresholds are age-stratified at 450 pg/mL under 50 years, 900 pg/mL for 50-75 years, and 1,800 pg/mL above 75 years, while BNP below 100 pg/mL is the corresponding rule-out value.
- Sacubitril/valsartan raises BNP because neprilysin inhibition blocks BNP degradation, but does not raise NT-proBNP, so NT-proBNP is the peptide to follow in any patient on an angiotensin receptor-neprilysin inhibitor. Obesity falsely lowers both.
- Digoxin levels must be drawn at least 6 to 8 hours after the dose, and the therapeutic range in heart failure is 0.5-0.9 ng/mL; a level drawn during the distribution phase reads falsely high and leads to inappropriate dose reduction.
Cardiac Troponin
Test-plan item IV.A.4 covers laboratory testing, and troponin is the single most examinable analyte in cardiac nursing.
What Makes an Assay High-Sensitivity
An assay qualifies as high-sensitivity cardiac troponin (hs-cTn) when it detects measurable troponin in at least half of a healthy reference population and achieves a coefficient of variation of 10% or less at the 99th percentile. Results are reported in ng/L as whole numbers, not in ng/mL — mixing the units by a factor of 1,000 is a recurring error on charts and on exams.
Sex-Specific Upper Reference Limits
The diagnostic threshold is the 99th percentile upper reference limit (URL) of a normal population, and it is assay-specific and sex-specific, because women have lower circulating troponin concentrations and smaller left ventricular mass. Representative values: one widely used hs-cTnI assay reports an overall URL near 26 ng/L with sex-specific limits of about 16 ng/L for women and 34 ng/L for men, while a common hs-cTnT assay uses an overall URL of 14 ng/L with sex-specific values of roughly 9 ng/L for women and 15-16 ng/L for men. Always use your own laboratory's cutoffs. Applying a single overall cutoff systematically under-diagnoses myocardial infarction in women, which is one reason women present later and are revascularised less often.
The Delta Concept and Rapid Algorithms
A single value is a snapshot; the diagnosis of acute myocardial injury requires a rising or falling pattern with at least one value above the 99th percentile URL. The change is judged as an absolute delta in ng/L, not as a percentage, because absolute change performs better at the low concentrations high-sensitivity assays detect. An elevated but essentially unchanging value is chronic myocardial injury.
0/1-hour and 0/2-hour algorithms stratify patients into three arms after a baseline and one repeat draw:
- Rule out — a very low baseline value (for one common hs-cTnT assay, below 5 ng/L with symptom onset more than 3 hours earlier), or a low baseline with a minimal 1-hour delta
- Rule in — a markedly elevated baseline, or a large 1-hour delta
- Observe — everything in between, requiring a 3-hour value and usually imaging or a stress evaluation
The specific numbers are assay-specific and are printed on the institutional pathway. The 2025 ACC/AHA/ACEP/NAEMSP/SCAI acute coronary syndrome guideline recommends high-sensitivity troponin as the preferred biomarker, used in a validated serial pathway alongside the ECG and clinical assessment, and recommends against routine CK-MB and myoglobin.
Injury Is Not Infarction
This is the most heavily tested idea in the whole laboratory topic. Myocardial injury is troponin above the 99th percentile. Myocardial infarction is injury plus clinical evidence of ischemia. Non-ACS causes of an elevated troponin the CMC nurse must be able to list:
| Mechanism | Examples |
|---|---|
| Supply-demand mismatch (type 2 MI) | Sustained tachyarrhythmia, severe hypotension or hypertensive emergency, profound anemia, hypoxemia, coronary spasm, spontaneous coronary artery dissection or embolism |
| Direct myocardial injury | Myocarditis, Takotsubo syndrome, cardiac contusion, chest compressions, cardiotoxic chemotherapy |
| Increased wall stress | Acute and chronic heart failure, hypertensive crisis |
| Right ventricular strain | Pulmonary embolism, severe pulmonary hypertension, ARDS |
| Systemic illness | Sepsis and critical illness, stroke and subarachnoid hemorrhage, rhabdomyolysis, cardiac amyloidosis |
| Reduced clearance and chronic injury | Chronic kidney disease, where hs-cTnT is frequently elevated at baseline and only the delta is interpretable |
| Procedural | Post-PCI, post-CABG, post-ablation, and post-cardiac surgery. Electrical cardioversion alone rarely produces a meaningful rise |
The Universal Definition of Myocardial Infarction
| Type | Definition |
|---|---|
| Type 1 | Atherothrombotic — plaque rupture or erosion with intraluminal thrombus |
| Type 2 | Supply-demand mismatch without acute atherothrombosis |
| Type 3 | Cardiac death with symptoms and ECG changes suggestive of ischemia, occurring before biomarkers could be obtained |
| Type 4a / 4b / 4c | PCI-related (troponin above 5 times the URL), stent thrombosis, and restenosis |
| Type 5 | CABG-related (troponin above 10 times the URL) |
Legacy Markers
CK-MB retains a narrow niche. Because it returns to baseline within 48 to 72 hours while troponin remains elevated for 5 to 14 days, a re-rise in CK-MB is interpretable for suspected reinfarction in the first few days after an index event, and some periprocedural and surgical protocols still track it. Myoglobin rises early but is released by all skeletal muscle and has essentially no current role.
Natriuretic Peptides
ProBNP is cleaved into the biologically active B-type natriuretic peptide (BNP), with a half-life of about 20 minutes, and the inactive N-terminal proBNP (NT-proBNP), with a half-life of roughly 70 to 120 minutes and predominantly renal clearance. Both are released in response to ventricular wall stress, which is why they track filling pressures rather than ejection fraction.
Diagnostic Cutoffs
| Peptide | Rule out acute heart failure | Rule in acute heart failure |
|---|---|---|
| BNP | Below 100 pg/mL | Above 400-500 pg/mL (100-400 is a grey zone) |
| NT-proBNP | Below 300 pg/mL at any age | Age-stratified: above 450 pg/mL under 50 years, above 900 pg/mL at 50-75 years, above 1,800 pg/mL over 75 years |
For chronic or ambulatory heart failure the diagnostic thresholds are far lower — BNP of 35 pg/mL or more, or NT-proBNP of 125 pg/mL or more, supports the diagnosis in a patient with compatible symptoms.
Confounders the Nurse Must Recognise
Falsely elevated without heart failure: advanced age, female sex, renal impairment (especially for NT-proBNP, whose clearance is renal), atrial fibrillation, pulmonary embolism and other causes of right ventricular strain, pulmonary hypertension, sepsis and critical illness, high-output states, severe anemia, and cardiotoxic chemotherapy.
Falsely low despite real heart failure: obesity is the classic answer — natriuretic peptide concentrations fall roughly by half at a body mass index above 35, so a BNP of 80 pg/mL in a severely obese patient does not exclude decompensation. Also flash pulmonary edema sampled very early before the peptide rises, constrictive pericarditis and tamponade (where the pressures are high but ventricular wall stress is not), and heart failure with preserved ejection fraction, which generally runs lower values than HFrEF.
The ARNI Rule
Sacubitril inhibits neprilysin, the enzyme that degrades BNP. BNP therefore rises and stays elevated on sacubitril/valsartan, while NT-proBNP is not a neprilysin substrate and remains valid. In a patient on an angiotensin receptor-neprilysin inhibitor, follow NT-proBNP, and never interpret a rising BNP as decompensation without checking the medication list. This is one of the highest-yield single facts in the CMC laboratory content.
Natriuretic peptides are recommended for diagnosis and for prognosis, including a pre-discharge value, but serial peptide-guided titration of medical therapy did not improve outcomes in randomised testing and is not a treatment target.
The Supporting Panel
| Test | Values and use in the cardiac patient |
|---|---|
| Lipid panel | A non-fasting sample is acceptable for screening and routine risk assessment; fast when triglycerides exceed about 400 mg/dL or a calculated LDL is required. LDL below 70 mg/dL is the established target in ASCVD, with below 55 mg/dL used for very-high-risk patients |
| Hemoglobin A1c | Generally below 7% with individualisation; falsely low with hemolysis, recent transfusion or advanced CKD, falsely high in iron deficiency |
| Renal panel and eGFR | Creatinine, eGFR by the 2021 race-free CKD-EPI equation, and a BUN-to-creatinine ratio above 20:1 suggesting prerenal physiology or low forward flow. Drives diuretic, contrast, ACE inhibitor and anticoagulant decisions |
| Hepatic panel | Acute cardiogenic shock produces shock liver with transaminases in the hundreds to thousands that fall quickly once perfusion is restored; chronic right-sided congestion produces a cholestatic pattern with raised alkaline phosphatase, GGT and direct bilirubin. MELD-XI is used in advanced heart failure and VAD or transplant candidacy |
| Lactate | Normal below 2 mmol/L; 4 mmol/L or more marks severe shock, and a clearance of at least 10% over 2 hours is a resuscitation target. Non-hypoxic causes: epinephrine and high-dose beta-2 agonists (glycolysis, not hypoperfusion), metformin, liver failure, thiamine deficiency, seizures, propofol infusion syndrome |
| PT/INR | Warfarin target 2.0-3.0; 2.5-3.5 for a mechanical mitral or older-generation valve |
| aPTT / anti-Xa | Unfractionated heparin titrated to an institution-calibrated aPTT, generally 1.5-2.5 times control; anti-Xa of 0.3-0.7 IU/mL is more reliable with lupus anticoagulant, high factor VIII, or aPTT-heparin discordance. Low-molecular-weight heparin anti-Xa is drawn 4 hours post dose |
| ACT | Bedside: roughly 250-300 seconds during PCI (lower with a glycoprotein IIb/IIIa inhibitor), above 400-480 seconds on cardiopulmonary bypass, and 180-220 seconds on many ECMO protocols |
| Thromboelastography / ROTEM | R time reflects clotting factors (give plasma), K time and alpha angle reflect fibrinogen (give cryoprecipitate), maximum amplitude reflects platelet function (give platelets or desmopressin), LY30 reflects fibrinolysis (give tranexamic acid). Valuable post-cardiotomy and on mechanical support |
| D-dimer | High negative predictive value only; use the age-adjusted cutoff (age times 10 ng/mL FEU above age 50) in low or intermediate probability. Elevated by surgery, malignancy, pregnancy, sepsis, aortic dissection and advanced age |
| Procalcitonin | Supports bacterial infection over a purely cardiogenic cause of shock or dyspnea and guides antibiotic discontinuation, but rises non-specifically after cardiopulmonary bypass, cardiac arrest and major surgery |
| Thyroid function on amiodarone | Baseline then approximately 6-monthly TSH and free T4, plus liver enzymes and chest imaging. Amiodarone-induced thyrotoxicosis type 1 (iodine-driven excess synthesis) is treated with a thionamide; type 2 (destructive thyroiditis) with corticosteroids. Amiodarone-induced hypothyroidism is treated with levothyroxine without stopping the drug |
| Digoxin level | Therapeutic 0.5-0.9 ng/mL in heart failure; the older 0.8-2.0 range is obsolete and higher levels increase mortality. Draw at least 6 to 8 hours after the dose, ideally 8-12 hours or immediately before the next dose. Toxicity is potentiated by hypokalemia, hypomagnesemia, hypercalcemia, hypothyroidism, renal impairment, amiodarone, verapamil, quinidine and macrolides |
| Electrolytes | Potassium 3.5-5.0 mEq/L, kept at 4.0-5.0 in patients on diuretics or digoxin; magnesium 1.8-2.4 mg/dL, kept above 2.0 with arrhythmia or a prolonged QT; ionized calcium 1.1-1.3 mmol/L, which falls with massive transfusion and with citrate-based regional anticoagulation on CRRT and causes hypotension and reduced contractility |
| LDH and plasma free hemoglobin | The LVAD hemolysis pair. LDH above roughly 2.5-3 times the upper limit is an early and sensitive marker of pump thrombosis; plasma free hemoglobin above 20-40 mg/dL confirms hemolysis. Tea-coloured urine with a rising LDH is escalated immediately |
| hs-CRP | Above 2 mg/L identifies residual inflammatory risk in stable ASCVD. It is non-specific and has no role in diagnosing acute coronary syndrome |
Mixed venous and central venous oxygen saturations are covered with hemodynamic monitoring elsewhere in this guide; interpret them alongside lactate rather than in isolation.
Pre-Analytic Error Is a Nursing Responsibility
Most erroneous laboratory results on a cardiac unit are created at the bedside, not in the analyser. These are directly examinable because they are nursing actions.
Hemolysis and Pseudo-Hyperkalemia
A hemolyzed specimen releases intracellular potassium and reports a falsely high potassium, along with a falsely high LDH, AST and magnesium and a falsely low sodium. Causes are small-bore needles, prolonged tourniquet time, fist clenching, drawing through a narrow catheter with excessive vacuum, vigorous mixing, and pneumatic tube transport.
The rule: a critical potassium in a patient who is asymptomatic, has a normal ECG, and has a laboratory comment of hemolysis is redrawn before treatment. But no high potassium is ever dismissed without obtaining an ECG first — peaked T waves, a widened QRS or a lost P wave override any suspicion of artifact.
Drawing Above an Infusing Line
Sampling proximal to (above) a running infusion draws infusate back into the specimen. The consequences are specific and predictable:
- Potassium-containing fluid gives a falsely high potassium
- Dextrose-containing fluid or parenteral nutrition gives a falsely high glucose and a falsely low potassium by dilution
- A heparin infusion gives a grossly prolonged aPTT in a patient who may be perfectly therapeutic
- Any infusion dilutes hemoglobin, hematocrit and all analytes
Draw from the opposite extremity. If that is impossible, stop the infusion for at least 2 minutes, draw distal to the site, and discard an adequate waste volume — then document what was done.
Arterial Line Sampling
Waste 3 to 6 mL before collecting, or use a closed in-line reservoir system and return the blood. Inadequate waste means the sample is a mixture of blood and flush solution, producing a falsely low hematocrit and potassium, a diluted glucose, and, because the flush may be heparinised, a prolonged aPTT or ACT. Closed systems also reduce iatrogenic blood loss, which matters in a patient already anemic from repeated draws.
Tube Fill and Order of Draw
- The light-blue citrate tube must be filled completely. The tube is designed for a 9:1 blood-to-citrate ratio; an underfilled tube leaves excess citrate that binds the calcium the assay needs and produces a falsely prolonged PT and aPTT. The same falsely prolonged result occurs when the hematocrit exceeds 55% unless a volume-adjusted tube is used.
- Order of draw: blood cultures, then light-blue citrate, then serum (red or gold), then green heparin, then lavender EDTA, then gray fluoride/oxalate. Drawing lavender before green or gold carries EDTA over into the next tube, producing a falsely low calcium and a falsely high potassium — a combination that can trigger an entirely unnecessary treatment cascade in a cardiac patient.
Timing Relative to Dosing
| Analyte | Correct timing |
|---|---|
| Digoxin | At least 6-8 hours after the dose; ideally just before the next dose |
| Low-molecular-weight heparin anti-Xa | 4 hours after a subcutaneous dose (peak) |
| Vancomycin trough | Within 30 minutes before the next dose |
| Cardiac troponin | Timed from symptom onset, not from arrival, and repeated per the institutional 0/1-hour or 0/2-hour pathway |
| Amiodarone thyroid panel | Baseline before initiation, then approximately every 6 months |
A digoxin level drawn 2 hours after a dose samples the distribution phase, when serum concentration greatly exceeds tissue concentration, and reads spuriously high. Reducing the dose on that number under-treats the patient.
Biomarker Time Course
| Marker | Begins to rise | Peaks | Returns toward baseline | Primary use |
|---|---|---|---|---|
| High-sensitivity cardiac troponin I or T | 1-3 hours | 12-48 hours | 5-14 days | Diagnosis of myocardial injury and infarction using serial deltas |
| Conventional cardiac troponin | 3-6 hours | 12-24 hours | 7-10 days | Superseded by high-sensitivity assays |
| CK-MB | 3-6 hours | 12-24 hours | 48-72 hours | Suspected reinfarction within days of an index event; some periprocedural protocols |
| Myoglobin | 1-3 hours | 6-9 hours | 24 hours | Historic; too non-specific for current use |
| BNP | Within hours of rising wall stress | Tracks filling pressure | Half-life about 20 minutes | Diagnosis and prognosis in heart failure; rises on sacubitril/valsartan |
| NT-proBNP | Within hours | Tracks filling pressure | Half-life 70-120 minutes, renally cleared | Diagnosis and prognosis; the peptide to follow on an ARNI |
| Lactate | Minutes | Reflects current perfusion | Clears as perfusion is restored | Shock severity and resuscitation adequacy |
| LDH with plasma free hemoglobin | Days to weeks in evolving pump thrombosis | — | — | LVAD hemolysis surveillance |
A patient with heart failure with reduced ejection fraction was started on sacubitril/valsartan three weeks ago during an admission for volume overload. Her BNP has risen from 620 to 940 pg/mL, while her weight is down 6 kg, her jugular venous pressure has fallen, and she now walks the unit hallway without dyspnea. What is the best interpretation?
A 74-year-old with stage 4 chronic kidney disease presents with vague chest discomfort. High-sensitivity cardiac troponin T is 62 ng/L at presentation and 65 ng/L one hour later, against a 99th-percentile upper reference limit of 14 ng/L. The ECG is unchanged from a tracing six months ago. What does this pattern most likely represent?
A patient receiving a heparin infusion through a left antecubital IV has an aPTT drawn from a left forearm vein proximal to the infusion site. The result returns at 180 seconds; the value six hours earlier was 62 seconds, and the patient has no bleeding and no change in condition. What should the nurse do first?