FROM MY PT SCHOOL NOTES · CARDIOPULMONARY · CHAPTER 5
What Do PFTs, Blood Gases and Diagnostic Tests Actually Tell You?
The goal isn’t to memorize every laboratory and imaging test. It’s to know what question the test answers—and what it cannot prove.
1. Separate spirometry from measured lung volumes
FEV1 measures the volume exhaled during the first second of a forced expiration, while FVC is the volume expelled during the forced maneuver. Their ratio helps identify airflow obstruction, but a reduced ratio is not itself a diagnosis of COPD or asthma.
Measured total lung capacity (TLC) is needed to confirm restriction. A reduced FVC alone may reflect restriction, gas trapping, poor technique or a nonspecific finding. Always consider whether the test was technically acceptable.
A person has FEV1 = 1.5 L and FVC = 2.5 L. What does 0.60 tell us?
Reasoning: The reduced ratio suggests airflow obstruction if the maneuver is technically sound. It does not identify the disease or establish COPD without further context and relevant post-bronchodilator data.
2. Oxygen saturation isn’t the whole gas-exchange picture
Pulse oximetry estimates peripheral oxygen saturation; arterial blood gases add information about oxygen tension (PaO2), carbon dioxide (PaCO2), pH and bicarbonate. Ventilation and oxygenation are related but different problems.
A reassuring SpO2 does not rule out elevated CO2, a metabolic disturbance or poor overall circulation. Likewise, motion or poor signal quality can lead to unreliable oximetry values. Interpret results with the person, the equipment and the rest of the examination.
Can a normal-looking SpO2 prove the patient is not retaining CO2?
Reasoning: No. Pulse oximetry does not directly measure PaCO2 or acid-base balance.
3. Match the test to the PT decision
Cardiopulmonary diagnostic testing may include pulmonary function tests, exercise tests, echocardiography, chest imaging, blood testing and vascular studies. A PT often uses this information rather than performing every investigation.
Historical slides sometimes conflate test sensitivity with the probability that a positive finding is correct. Keep the definitions straight: sensitivity = TP/(TP+FN); specificity = TN/(TN+FP); predictive values are different and depend in part on disease prevalence.
Does a highly sensitive test automatically mean any positive result is convincing?
Reasoning: No. Positive predictive value also depends on specificity and the prior likelihood of disease.
Apply it to a clinical situation
Scenario: A patient with a low FVC on spirometry is labeled ‘restrictive’ in a handoff. Should you consider that a confirmed restriction diagnosis?
What is the stronger clinical decision?
Reasoning: No. Review whether TLC was measured and the complete interpretation, including maneuver quality and possible air trapping, before treating the label as established.
Common trap: Using a single test value as a complete medical diagnosis.
Source notes & learning scope
Course sources: “Chapter 10.pdf” and “1 Cardiopulmonary testing Part 1.pdf.” ERS/ATS interpretation and GOLD 2026 clarify the update. An independent teaching synthesis, not a published protocol or clinical certification.
