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Do you know how to interpret the results of your spirometry test?

Do you know how to interpret the results of your spirometry test?

Hello world! This is the second of our articles about spirometry and its interpretation. In this article, we'll focus on helping you to understand what the results of your spirometry test mean. Many different types of measurements can result from a spirometry test, but this article will cover only the major ones. At first, all these numbers can look intimidating, but they are actually not difficult to understand. The three most common results you will see are FVC (forced vital capacity), FEV1 (forced expiratory volume in one second), and FEV1/FVC ratio. Some spirometers also produce a graph that visually represents the flow of air, measured against the total volume of air exhaled.

  • FVC – Forced Vital Capacity measures the total volume of air that you were able to blow forcefully into the mouthpiece following a full inhalation. The Measured column shows the total volume in liters. Average normal values in healthy males aged 20-60 range from 5.5 to 4.75 liters, and average normal values for females aged 20-60 range from 3.75 to 3.25 liters. The Predicted column compares the actual total volume breathed out during the test to an average of the normal total volume for a person of the same gender, height, and age. This is expressed as a percentage, with normal test values falling between 80% and 120% of the average (predicted) values.
  • FEV1 The Forced Expiratory Volume in 1 Second parameter measures the volume of air that was exhaled into the mouthpiece in the first second after a full inhalation. The Measured column represents the total volume exhaled during the first second, in liters. Normal values in healthy males aged 20-60 range from 4.5 to 3.5 liters, and normal values for females aged 20-60 range from 3.25 to 2.5 liters. The Predicted column compares the actual volume breathed out during the first second of your test to an average of the normal volume breathed out in 1 second for a person of the same gender, height, and age. This value is expressed as a percentage, with normal test values being between 80% and 120% of the average (predicted) values.
  • RatioThe FEV1/FVC Ratio (FEV1%) parameter is calculated by dividing the measured FEV1 value by the measured FVC value. The Measured column shows the absolute (numerical) ratio, and the Predicted column shows the ratio expressed as a percentage. In healthy adults of the same gender, height, and age, the normal Predicted percentage should be between 70% and 85%. Percentages lower than 70% are considered abnormal. This is an important measurement because obstructive diseases such as, COPD, chronic bronchitis, and emphysema cause increased airway resistance to expiratory airflow, and may result in percentages of 45% to 60%. Restrictive diseases such as pulmonary fibrosis tend to reduce both FEV1 and FVC values, so the percentage can remain within the normal range, or even increase. See the final section below for more suggestions on assessing your result.
  • Flow / Volume graph This graph (called a spirogram) presents a visual record of the expiration portion of the test, with breath Flow (measured in liters per second) shown along the X (left) axis, and breath Volume (measured in liters) shown against the Y (bottom) axis. Interpreting these graphs may require some assistance from your doctor at first, but in general you should expect to see an instantaneous jump at the start of expiration, rising steadily to a sharp peak early in the expiration period, and followed by a slow, smooth fall in flow, without interruptions. Characteristics of the chart that could indicate respiratory problems (or problems during the test such as coughing) include:
    • A slow start, followed by a slow rise in flow
    • A late peak in flow
    • An erratic flow, with multiple peaks and troughs
    • An abrupt return to zero flow
  • DurationThe total expiration (exhalation) time for a spirometry test, measured in seconds. Six seconds is considered a standard duration for these tests, so for the best and most accurate results, you should try to exhale for a minimum of six seconds, even if you run out of breath sooner. Healthy individuals may not be able to exhale for a full six seconds, and this is not a problem because they are able to empty their lungs more rapidly than people with respiratory conditions.

Evaluating your spirometry test results

There are many methods of interpreting the results of spirometry tests. The following 5-step approach is commonly used to detect the presence of respiratory disease:

  1. Start by looking at the FVC parameter to see if it falls within the normal range.
  2. Next, look at the FEV1 parameter to see if it's within the normal range.
  3. If both the FVC and FEV1 values are normal, in all likelihood the results of your spirometry test can be considered normal.
  4. If either the FVC and FEV1 values are below the predicted normal value, there is a possibility of lung disease and you should go on to step 5.
  5. Look at the predicted FEV1/FVC Ratio value. If it is 69% or less, there is a strong possibility that you have some form of restrictive lung disease.

In general, your predicted percentages for FVC and FEV1 should be above 80% and your FEV1/FVC Ratio percentage should be above 70% to be considered normal.

However, the information provided in these spirometry results can be used in many additional ways. For example, when treating COPD, FEV1 is often measured after the patient has been given a bronchodilator medication to open their airways, and the resulting values are used to grade the severity of the patient's disease:

  • 80% or more – mild COPD (able to achieve normal results after medication)
  • 50-79% – moderate COPD
  • 30-49% – severe COPD
  • less than 30% – very severe COPD

Naturally, you should check with your doctors to see if there is an approach to interpreting spirometry results that they prefer, and if there are particular parameters that you should be watching to make sure your condition is improving.

3 comments

Aug 04, 2017 • Posted by NuvoAir

Hi there,
the reference values reported used to calculate %Pred are Nhanes III.
Best wishes,
NuvoAir Team

Jul 16, 2017 • Posted by Sara Cousillas

Hello,
Could you please tell me which set of reference values are used for calculating the %PRED?

Thanks!

May 02, 2017 • Posted by sarah

Hi there! I have been trying out this device and loving it! I am comparing with one I use at work. Could you please advise which set of reference values are used for calculating the %PRED?

Thanks!

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