Diagnostic tests

Blood tests

Tests to identify myocardial infarction

After a myocardial infarction (MI), damaged cardiac tissue releases significant amounts of enzymes and proteins into the blood. Specific blood tests help confirm the diagnosis of MI, reveal the extent of cardiac damage, help monitor healing progress, and assess reperfusion after the use of fibrinolytics.

Serum cardiac biomarkers

Myoglobin
- Elevated
- First marker of cardiac injury after acute MI
- Found in skeletal muscle
Creatine kinase (CK) and CK-MB
- Both return to normal quickly
- CK-MB most reliable when reported as a percentage of total CK (relative index)
- Found in cardiac muscle (CK-MB) and skeletal muscle (CK)
Troponin I
- Isotype of troponin found only in myocardium
- Elevated
- Specific to myocardial damage
Troponin T
- Isotype of troponin that’s less specific to myocardial damage (can indicate renal failure)
- Elevated
- Determined quickly at bedside

Values

Myoglobin

Normal value: 0 to 0.09 mcg/ml
Rises within 30 minutes to 4 hours
Peaks within 6 to 10 hours
Returns to baseline by 24 hours

CK-MB

Normal value: 38 to 190 units/L for men; 10 to 150 units/L for women
Rises within 4 to 8 hours
Peaks in 12 to 24 hours
May remain elevated for up to 96 hours

Troponin I

Normal value: Less than
0.4 mcg/ml (may vary depending on the laboratory)
Rises within 4 to 6 hours
Peaks in 12 hours
Returns to baseline in 3 to 10 days

Troponin T

Normal value: Less than 0.1 mcg/ml (may vary depending on the laboratory)
Rises within 4 to 8 hours Peaks in 12 to 48 hours
Returns to baseline in 7 to 10 days

Tests to identify the risk of heart disease

Homocysteine (tHcy)

Normal value: < 13 umol/L
Excess levels
- Irritate blood vessels, leading to atherosclerosis
- Raise low-density lipoprotein (LDL) levels
- Make blood clot more easily

High-sensitivity C-reactive protein (hs-CRP)

Normal value: 0.1 to 0.3 mg/dl
Excess levels: May indicate increased risk of coronary artery disease (CAD)

Triglycerides

Normal value: < 150 mg/dl
Excess levels: Help with early identification of hyperlipidemia and identification of patients at risk for CAD

Total cholesterol

Normal value: < 200 mg/dl for adults; < 170 mg/dl for children and adolescents
Excess levels: May indicate hereditary lipid disorders, CAD

Evaluating lipid test results

Use this chart to determine an adult patient’s risk of CAD.

Lipoprotein fractionation

Lipoprotein fractionation tests isolate and measure high-density lipoproteins (HDLs), LDLs, and very-low-density lipoproteins (VLDLs). Each of these particles is composed of protein, cholesterol, and triglyceride in varying amounts.

HDL

Primarily protein
Test measures the actual amount in the blood
The higher the level, the lower the risk of CAD
Normal values for males: 35 to
65 mg/dl; for females, 35 to 80 mg/dl

LDL

Mainly cholesterol
Equal to total cholesterol – HDL cholesterol – VLDL cholesterol (when triglyceride level is below 400 mg/dl)
The higher the LDL level, the higher the risk of CAD
Normal levels for individuals without CAD, < 130 mg/dl
Optimal levels for individuals with CAD, < 100 mg/dl

VLDL

Mainly triglycerides
Calculated as triglyceride level ÷ 5
The higher the VLDL level, the higher the risk of CAD
Can be measured with a more sensitive test when high-risk patients and those with triglyceride levels of 400 mg/dl or more require complex medical management

Tests to identify the risk of heart failure

Heart cells produce and store two neurohormones — A-type natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) — that help ensure cardiac equilibrium. Disruptions in fluid balance within the circulatory system trigger release of these hormones, which act as natural diuretics and antihypertensives.

ANP

Found in atrial tissue
Normal value: 20 to 77 pg/ml

BNP

Found in ventricular tissue
Helps accurately diagnose and grade heart failure severity
Normal value: < 100 pg/ml

Correlating the degree of heart failure with I3NP level

The higher a patient’s level of BNR the greater the degree of heart failure. In turn, the greater the degree of heart failure, the more the patient’s ability to perform activities of daily living (ADLs) will be impaired. Use this chart to help you plan your nursing care.

New York Heart Association Classification

Tests for general screening

General screening tests are used to evaluate overall health and response to treatment.

Electrolyte tests

Electrolytes — which occur in the fluids both inside and outside cells — are crucial for nearly all cellular reactions and functions.

Potassium, calcium, magnesium

Sodium, chloride, carbon dioxide

▲ Potassium

Most critical value
Has narrow therapeutic range
Imbalances cause life-threatening arrhythmias
Affected by diuretics, penicillin G, some nonsteroidal anti-inflammatory drugs.

● Calcium

High values cause cardiac toxicity and arrhythmias
Elevations commonly caused by cancer or hyperparathyroidism

Magnesium

High values cause ECG changes, ventricular tachycardia, and ventricular fibrillation
Low values cause ECG changes, bradycardia, and hypotension

Sodium

Maintains osmotic pressure, acid-base balance, and nerve impulse transmission
Levels decreased in severe heart failure
Decreased by diuretics, high triglycerides, and low blood protein

Chloride

Partners with sodium to maintain fluid and acid-base balance
Low levels in heart failure and metabolic acidosis

Carbon dioxide

Primarily made up of bicarbonate
Regulated by the kidneys
Levels lowered by thiazide diuretics

Coagulation teste

Activated partial thromboplastin time (aPTT), prothrombin time (PT), bleeding time, and activated clotting time (ACT) are tests that measure clotting time. They’re used to measure response to treatment as well as to screen for clotting disorders.

Understanding clotting

Clotting is initiated through two different pathways.

Common tests for clotting

Test	Action	Clinical uses	Location performed	Normal range	Therapeutic ranges	Panic value
ACT	Measures overall coagulation activity	♥ Evaluates effects of high dose heparin therapy during cardiac procedures	Bedside	70 to 120 seconds	2 times normal range	Unknown
Bleeding time	Determines platelet function abnormalities	♥ Screens for platelet abnormalities before or during surgery ♥ Used to diagnose von Willebrand’s disease, vascular disorders, hemostatic dysfunctions	Bedside	3 to 10 minutes	Unknown	> 15 minutes
aPTT	Measures defects in intrinsic and common clotting pathways	♥ Evaluates effects of heparin therapy ♥ Assesses overall coagulation system	Laboratory	21 to 35 seconds	2 to 2.5 times normal range	> 70 seconds
PT	Directly measures deficits in extrinsic and common clotting pathways	♥ Evaluates effects of coumar therapies ♥ Assesses for vitamin K deficiency ♥ Used to diagnose liver failure	Laboratory	11 to 13 seconds	2 to 2.5 times normal range	> 30 seconds