5 Blood Gases, Cardiac Markers, and Clinical Practice

Interpret results in clinical context

Laboratory results are clues, not diagnoses in isolation. Interpret them alongside symptoms, trends, vital signs, oxygen or ventilator settings, the ECG, and the patient’s baseline. Use local reference ranges and critical-result policies.

A deteriorating patient needs assessment and escalation; do not wait for a laboratory result to act.

What arterial blood gases measure

An Arterial blood gas (ABG) helps assess acid–base balance, ventilation—the removal of carbon dioxide—and oxygenation. Common adult reference ranges at or near sea level are listed below; laboratory ranges may differ. PaO₂ depends on factors such as altitude and supplemental oxygen.

• pH: $7.35$–$7.45$. Below $7.35$ indicates acidemia; above $7.45$ indicates alkalemia.

• PaCO₂: $35$–$45$ mmHg. This reflects the respiratory component; CO₂ acts as an acid.

• HCO₃⁻: $22$–$26$ mEq/L. This reflects the primarily renal metabolic component; bicarbonate acts as a base.

• PaO₂: $75$–$100$ mmHg. This reflects oxygen dissolved in arterial blood.

• SaO₂: about $95$–$100$%. This is the percentage of hemoglobin carrying oxygen.

A practical ABG interpretation sequence

Interpret the gas alongside the patient’s condition and the oxygen support in place when the sample was taken. Note work of breathing, mental status, perfusion, SpO₂, and oxygen or ventilator settings.

Read the pH first to determine whether the blood is acidemic, alkalemic, or within the reference range. Then look for a primary pattern: in respiratory disorders, pH and PaCO₂ generally move in opposite directions; in metabolic disorders, pH and HCO₃⁻ generally move in the same direction.

For example, low pH with high PaCO₂ suggests respiratory acidosis. Low pH with low HCO₃⁻ suggests metabolic acidosis.

Next, assess Compensation and consider whether a mixed disorder may be present. The other system may shift to reduce the pH change, but compensation does not necessarily return pH to normal. A near-normal pH alongside abnormal PaCO₂ and HCO₃⁻ does not by itself prove one fully compensated disorder. Compare the response with what is expected and with the clinical context; an unexpected response can indicate a mixed disorder.

Evaluate oxygenation separately. A near-normal pH does not rule out dangerous hypoxemia. Interpret PaO₂ with oxygen delivery, SpO₂, and the clinical picture.

ABG patterns in clinical scenarios

A patient with COPD becomes drowsy and has increased work of breathing. The ABG shows pH $7.28$, PaCO₂ $60$ mmHg, HCO₃⁻ $27$ mEq/L, and PaO₂ $58$ mmHg.

Acidemia with elevated PaCO₂ indicates respiratory acidosis; the slightly elevated HCO₃⁻ is consistent with some metabolic compensation. PaO₂ is low. The change in alertness and hypoxemia make this urgent: assess airway and breathing, verify oxygen delivery and monitoring, and promptly escalate to the responsible clinician or rapid-response team according to the patient’s condition and facility policy. Do not assume the result is simply the patient’s baseline COPD.

A patient with suspected diabetic ketoacidosis has pH $7.25$, PaCO₂ $26$ mmHg, and HCO₃⁻ $12$ mEq/L. Acidemia with low HCO₃⁻ indicates metabolic acidosis; low PaCO₂ is consistent with respiratory compensation. Assess the patient’s overall condition and related results, including glucose, ketones, electrolytes, and perfusion, and escalate promptly. Worsening mental state, shock, or increasing respiratory distress requires immediate attention.

Cardiac troponin and suspected ACS

High-sensitivity cardiac troponin (cTnI or cTnT) is the preferred biomarker for detecting Myocardial injury in suspected acute coronary syndrome (ACS). A value above the assay-specific upper reference limit indicates myocardial injury, but one elevated value alone does not establish the cause or prove a heart attack.

Interpret troponin with symptoms, ECG findings, and serial results. A rise or fall with evidence of ischemia supports acute myocardial infarction. Use the assay’s own cutoffs and the ordered testing pathway: values and meaningful changes are not interchangeable across assays. Early results may be nondiagnostic, so repeat testing may be required. When the initial high-sensitivity result is nondiagnostic, the 2025 ACC/AHA guideline recommends repeat sampling at $1$–$2$ hours; for conventional assays, it recommends repeat sampling at $3$–$6$ hours.

With ECG evidence of STEMI, do not delay reperfusion treatment while waiting for troponin results. For suspected ACS, obtain or assist with prompt ECG evaluation and report persistent or worsening symptoms immediately.

Other cardiac biomarkers

BNP and NT-proBNP are released in response to cardiac wall stress. They can support assessment for heart failure, especially when a patient has shortness of breath, but must be interpreted with the examination and other tests. An elevated value is not specific to heart failure, and BNP and NT-proBNP should not be compared as though they were the same test.

CK-MB is a less commonly used cardiac marker where cardiac troponin testing is available. If CK-MB appears in a result set, interpret it using local guidance and the full clinical picture rather than treating it as a substitute for troponin.

Interpreting cardiac markers in context

A patient reports new chest pressure and nausea. The first high-sensitivity troponin is below the assay’s upper reference limit, but symptoms continue. One early result does not safely exclude evolving myocardial injury. Promptly communicate the symptoms, ensure timely ECG evaluation, and follow the ordered serial-troponin pathway. Escalate immediately if symptoms worsen, the patient becomes unstable, or the ECG is concerning. Do not reassure the patient or delay escalation based on one initial value.

A patient with shortness of breath has an elevated BNP. This supports considering heart failure but does not diagnose it by itself. Assess respiratory status, oxygenation, perfusion, edema, and relevant trends; promptly report signs of acute deterioration and share the result in clinical context.

Communicating a concerning result

Use SBAR to give a concise, structured report:

• Situation: State the immediate concern, such as new or worsening chest pressure and shortness of breath.

• Background: Give relevant history, symptom onset, baseline status, oxygen support, and pertinent medications or risks.

• Assessment: Report current vital signs, mental status, ECG findings if available, the exact laboratory value and units, the reference limit, and prior or repeat values with their times.

• Recommendation or request: State what is needed, such as urgent bedside assessment, clarification of orders, or activation of the escalation pathway. Read back critical instructions according to policy.

For a critical or unexpected result, verify patient and specimen details, promptly notify the appropriate clinician according to policy, document the notification and response, and reassess the patient. Treat changes in the patient—not just numbers—as the priority.

Putting the findings together

Interpret ABGs systematically: pH first, then PaCO₂ and HCO₃⁻, compensation, and oxygenation. ABG patterns help identify a disturbance but do not replace assessment of the patient or establish the cause.

Troponin detects myocardial injury; symptoms, ECG, assay-specific thresholds, and serial change help determine its significance. BNP and NT-proBNP can support heart-failure assessment but are not diagnostic alone.

Escalate promptly for instability, significant hypoxemia, worsening mental status, persistent chest symptoms, or critical results. Follow local protocols and do not delay urgent care while awaiting biomarkers.