Pulmonary Function Tests (PFTs)

A comprehensive guide to interpreting lung function tests, from simple spirometry to complex lung volumes and diffusion capacity.

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Sample PFT Report

⚠️ DISCLAIMER: This is an example report for educational purposes ONLY. It shows a *specific pattern* of moderate obstructive disease (COPD) and is NOT representative of all results. Do not use for self-diagnosis.

Pulmonary Function Panel

PATIENT: DOE, JOHN (ID: 12345)

STATUS: FINAL
DATE: 2025-10-20

Test	Result	Post-BD	Flag	Predicted	% Pred	LLN
SPIROMETRY
FVC (L)	3.80	3.85	LOW	4.22	90%	3.60
FEV1 (L)	2.10	2.15	LOW	3.80	55%	3.10
FEV1/FVC (%)	55	56	LOW	> 70	-	70
FEF 25-75% (L/s)	1.12	1.18	LOW	3.64	31%	2.80
LUNG VOLUMES (Plethysmography)
TLC (L)	7.80	-	HIGH	6.50	120%	7.50
RV (L)	3.50	-	HIGH	2.50	140%	3.10
RV/TLC (%)	45	-	HIGH	< 40	-	40
DIFFUSING CAPACITY
DLCO (mL/min/mmHg)	12.5	-	LOW	25.0	50%	19.8

IMPRESSION: Pattern shows a moderate obstructive lung disease (low FEV1, low FEV1/FVC ratio). There is no significant bronchodilator response. Lung volumes show hyperinflation (high TLC) and air trapping (high RV/TLC). The diffusing capacity (DLCO) is significantly reduced. These findings are classic for **COPD with an emphysematous component**.

Breathing by the Numbers: A Deep Dive into Pulmonary Function Tests (PFTs)

Learn to interpret spirometry, lung volumes, and diffusion capacity to diagnose conditions like COPD, asthma, and interstitial lung disease.

Beyond listening with a stethoscope, **Pulmonary Function Tests (PFTs)** are the primary tool used to objectively measure lung function. They are a non-invasive and highly informative set of tests that provide detailed numbers on how much air the lungs can hold, how quickly a patient can move air in and out, and how well the lungs transfer oxygen into the blood. For patients with respiratory symptoms like shortness of breath (dyspnea), coughing, or wheezing, PFTs are essential for diagnosing, quantifying the severity of, and monitoring the progression of lung diseases.

Interpreting a PFT report is a core skill for any clinician. It requires understanding a few key parameters and, most importantly, recognizing the classic **patterns** that define different categories of lung disease. The two most fundamental patterns to distinguish are **obstructive lung disease** (difficulty getting air *out*) and **restrictive lung disease** (difficulty getting air *in*). This guide will break down the components of a full PFT report, from basic spirometry to advanced measurements like lung volumes and diffusing capacity.

Part 1: Spirometry – The Core of PFTs

Spirometry is the most common and accessible PFT. It measures how much air you can forcefully exhale and how quickly you can do it. The patient is asked to take the deepest breath possible and then exhale as hard, fast, and completely as they can into a mouthpiece connected to a machine called a spirometer.

Key Spirometry Parameters:

FVC (Forced Vital Capacity): The *total volume* of air that a patient can forcefully exhale after taking their deepest possible breath. It's a measure of lung size. A low FVC suggests a *restrictive* problem (the lungs are small or stiff) or severe air trapping from an obstructive disease.
FEV1 (Forced Expiratory Volume in 1 Second): The *volume* of air exhaled in the **first second** of the FVC maneuver. This measures how *quickly* air can be moved out of the lungs. It is highly dependent on the caliber of the airways.
FEV1/FVC Ratio (or FEV1%): This is the **most important number** for distinguishing obstructive from restrictive disease. It represents the percentage of the total vital capacity that can be forcefully exhaled in the first second. A healthy person can typically exhale 70-80% or more of their air in the first second.
FEF 25-75% (Forced Expiratory Flow 25-75%):** The average flow rate during the middle portion of the FVC. It is thought to be a sensitive marker of disease in the *small* airways, but it is highly variable and less reliable than the FEV1/FVC ratio.

How to Read Spirometry: Predicted vs. Actual

PFT results are always compared to "predicted" values. These are normal values derived from large population studies, adjusted for the patient's **age, height, sex, and ethnicity**, as these factors all affect lung size. A result is typically reported as the actual value (e.g., FEV1 = 2.10 L) and the **percent predicted** (e.g., 55% of the 3.80 L that was predicted for this patient). Generally, values >80% of predicted are considered normal. The LLN (Lower Limit of Normal) is a more statistically precise cutoff, representing the 5th percentile for a healthy person of that demographic.

Part 2: The Two Great Patterns – Obstructive vs. Restrictive

Spirometry is primarily used to identify these two patterns.

1. The Obstructive Pattern (Difficulty Getting Air OUT)

The Problem:** Airflow is obstructed (blocked) or narrowed, making it difficult to exhale quickly. This is the hallmark of diseases like **COPD, asthma, and bronchiectasis**.

The Key Finding:** The FEV1/FVC ratio is **low** (classically < 70% or < LLN). The patient's total lung capacity (FVC) might be normal or reduced (due to air trapping), but the amount they can get out in one second (FEV1) is *disproportionately* reduced.

Severity (often based on FEV1 % Predicted):

Mild: FEV1 > 80% pred. (but ratio is low)

Moderate: FEV1 50-80% pred. (like the sample report)

Severe: FEV1 30-50% pred.

Very Severe: FEV1 < 30% pred.

2. The Restrictive Pattern (Difficulty Getting Air IN)

The Problem:** The lungs are "restricted" from fully expanding. This can be due to problems *inside* the lung (e.g., **interstitial lung disease (ILD)**, fibrosis, sarcoidosis) or *outside* the lung (e.g., morbid obesity, scoliosis, neuromuscular weakness like ALS).

The Key Finding:** The FEV1/FVC ratio is **normal or high** (> 70-80%). Both the FEV1 and FVC are *proportionally* reduced. The patient has small lungs, but the airways are not obstructed, so they can exhale a high percentage of their small volume very quickly.

Confirmation:** A true restrictive pattern must be confirmed by measuring **Lung Volumes**, specifically a low Total Lung Capacity (TLC).

Part 3: The Bronchodilator Response – Pinpointing Asthma

If an obstructive pattern is found, the spirometry test is repeated after the patient inhales a bronchodilator (like albuterol). This is a critical step to check for **reversibility**.

No Significant Response (e.g., COPD):** The FEV1 and/or FVC do not improve significantly. This indicates *fixed* airflow obstruction, as seen in COPD (emphysema/chronic bronchitis). The sample report shows this (FEV1 only improved from 2.10L to 2.15L).

Significant Response (e.g., Asthma):** A "significant" response is typically defined as an increase of **> 12% AND > 200 mL** in either the FEV1 or FVC. This indicates that the obstruction is reversible and is the classic diagnostic finding for **asthma**.

Part 4: Lung Volumes – Confirming Restriction & Air Trapping

Spirometry cannot measure all the air in the lungs. It can't measure the **Residual Volume (RV)**, which is the air left in the lungs *after* a maximal exhalation. To measure RV and the **Total Lung Capacity (TLC)**, more advanced tests are needed (like helium dilution or body plethysmography).

Restrictive Pattern:** As mentioned, a true restrictive pattern is *confirmed* by a **low TLC** (< 80% predicted or < LLN).

Obstructive Pattern:** In obstructive diseases, patients often can't get all the air out, leading to air trapping. This is seen as a **high RV** and often a **high TLC** (called hyperinflation). The sample report shows this classic pattern of air trapping, supporting the COPD diagnosis.

Part 5: Diffusing Capacity (DLCO) – Assessing Gas Exchange

The **DLCO** (Diffusing Capacity of the Lung for Carbon Monoxide) is a crucial test that measures how well gas (in this case, a tiny, safe amount of carbon monoxide) moves from the alveoli (air sacs) into the capillaries (bloodstream). It's a direct measure of the health and surface area of the alveolar-capillary membrane.

Interpreting the DLCO:

Low DLCO:** Indicates a problem with the alveolar membrane, either:

Destruction of Alveoli:** This is the classic finding in **Emphysema**. The lung volumes are large (hyperinflation), but the surface area for gas exchange is destroyed. This is why the sample report (Obstruction + High TLC + Low DLCO) points strongly to emphysema.

Thickening/Fibrosis:** In **Interstitial Lung Diseases (ILD)** (like pulmonary fibrosis), the membrane is thickened, scarring, or inflamed, impeding gas transfer. The PFT pattern for ILD is **Restrictive (low TLC) + Low DLCO**.

Other causes include pulmonary embolism and anemia (which must be corrected for).

Normal DLCO:** In an obstructive pattern, a normal DLCO suggests the obstruction is in the airways *without* alveolar destruction. This is typical of **Asthma** or **Chronic Bronchitis**.

High DLCO:** Can be seen in conditions with increased pulmonary blood flow, like early heart failure or alveolar hemorrhage.

Conclusion: A Complete Picture of Lung Health

The PFT panel is a powerful diagnostic tool. By systematically assessing spirometry (FEV1, FVC, ratio), bronchodilator response, lung volumes (TLC, RV), and diffusing capacity (DLCO), you can move beyond a simple "short of breath" complaint. You can confidently distinguish between obstructive and restrictive patterns, pinpoint the likely cause (Asthma vs. COPD/Emphysema vs. ILD), and quantify the disease's severity. This comprehensive understanding is the foundation for all pulmonary medicine and a vital skill for any medical scholar.

PFT FAQs

Your common questions about Pulmonary Function Tests, answered.

How should I prepare for a PFT? Do I need to fast?

Fasting is generally **not** required. However, the most important preparation is to **avoid using short-acting bronchodilators** (like your rescue inhaler, e.g., Albuterol/Salbutamol) for at least 4-6 hours before the test. You should also avoid smoking for at least one hour before and avoid heavy meals or vigorous exercise, as these can affect your ability to perform the maneuvers. Always follow the specific instructions from the lab.

What is the difference between Obstructive and Restrictive lung disease?

It's the simplest way to categorize lung diseases. **Obstructive** means you have trouble getting air **OUT**. Your airways are narrowed or blocked. The hallmark is a **low FEV1/FVC ratio** (< 70%). Examples include COPD, Asthma, and Bronchiectasis. **Restrictive** means you have trouble getting air **IN**. Your lungs are "restricted" and can't expand fully. The hallmark is a **low Total Lung Capacity (TLC)**. On basic spirometry, both FEV1 and FVC are low, but the FEV1/FVC ratio is *normal or high*. Examples include Pulmonary Fibrosis or Scoliosis.

What is a bronchodilator challenge, and why is it done?

If your initial spirometry shows an obstructive pattern (a low FEV1/FVC ratio), the technician will give you a dose of a bronchodilator (like Albuterol) and have you wait 10-15 minutes before repeating the test. This is done to check for **reversibility**. A significant improvement (increase of >12% AND >200mL in FEV1 or FVC) is the classic sign of **Asthma**. A lack of significant improvement suggests a fixed obstruction, which is typical of **COPD**.

Why is the FEV1/FVC ratio the most important number?

Because it's the primary indicator of **obstruction**. A patient can have a low FEV1 and a low FVC, but if the ratio of the two is normal, it's not an obstructive disease (it's likely restrictive). The ratio defines obstruction because it shows *disproportionate* trouble exhaling. A healthy person can get >70% of their air out in the first second. An obstructive patient might only get 50% out, even if their total lung size (FVC) is near normal.

What does the DLCO test measure?

DLCO (Diffusing Capacity) measures how well oxygen moves from your alveoli (air sacs) into your bloodstream. It tests the health of the thin alveolar-capillary membrane. A **low DLCO** is a very important finding. It means the membrane is damaged, either by **destruction** (as in **Emphysema**, where alveoli are lost) or by **thickening/scarring** (as in **Interstitial Lung Disease/Fibrosis**). It helps differentiate COPD-Emphysema from pure Chronic Bronchitis or Asthma (which typically have normal DLCO).