Total Lung Capacity (TLC) is the total amount of air your lungs can hold after your deepest breath. It is measured during lung function tests, also known as spirometry, to assess overall lung health and help diagnose certain conditions.
Purpose of Measuring Total Lung Capacity
Measuring total lung capacity helps diagnose and distinguish between two main types of lung conditions. The first is restrictive lung disease, where the lungs cannot fully expand, leading to a lower-than-normal TLC. This can be caused by conditions like pulmonary fibrosis (scarring of lung tissue) or scoliosis.
The second category is obstructive lung disease, which hinders a person’s ability to exhale completely. This can lead to air trapping in the lungs, resulting in a normal or even higher-than-expected TLC. Common examples include emphysema and chronic bronchitis, which are forms of Chronic Obstructive Pulmonary Disease (COPD).
The Measurement Process
A standard spirometry test, which measures how much and how quickly you exhale, cannot directly measure TLC. This is because it cannot account for the air remaining in your lungs after a full exhalation, known as the residual volume (RV). Therefore, more advanced techniques are required to calculate this remaining air and determine total lung capacity.
One common method is whole-body plethysmography, or the “body box” test. The patient sits inside a small, airtight booth and breathes into a mouthpiece. As they breathe, the device measures small pressure changes within the booth, which are used to precisely calculate the volume of air in the lungs and determine TLC.
Another method involves gas dilution tests, such as the helium dilution technique. This test involves breathing a mixture of air with a known concentration of helium. A machine then analyzes how much the helium is diluted by the air in the lungs to calculate the total lung volume.
A similar approach is the nitrogen washout test, where the patient breathes 100% oxygen to “wash out” the nitrogen from their lungs. Technicians measure the total amount of nitrogen exhaled to determine the initial lung volume. For all TLC measurement tests, the patient wears a nose clip to ensure breathing occurs only through the mouth for an accurate reading.
Understanding Your Results
Your results are interpreted by comparing them to predicted values based on your age, height, biological sex, and ethnicity. The final result is expressed as a percentage of this predicted value for standardized interpretation.
A low TLC is a value below 80% of the predicted normal. This result indicates the lungs are holding less air than expected and is a hallmark of restrictive lung disease, where lung expansion is limited.
A normal TLC is between 80% and 120% of the predicted value. This range is expected in healthy individuals but can also be present in mild obstructive lung disease. A normal TLC does not rule out a lung condition and is considered with other measurements.
A high TLC, a value above 120% of the predicted value, points toward lung hyperinflation. This means excess air is trapped in the lungs after exhalation and is an indicator of obstructive lung disease.
Preparing for the Test
To ensure accurate results, you will be instructed to avoid smoking for at least one hour before the test. You should also refrain from large meals or caffeinated beverages immediately before your appointment.
You may be asked to temporarily stop using certain inhaled breathing medications, such as short-acting bronchodilators, before the test. The exact timing varies, so follow the clinic’s specific instructions. Wearing loose, comfortable clothing is also recommended to ensure your breathing is not restricted.