Pulmonary Function Test Calculator

Calculator Inputs

Use the largest acceptable pre values. Post-bronchodilator values are optional. The form stays single-column overall, while fields adapt to screen size.

Example Data Table

Metric	Example Pre	Example Predicted	Example Post	Comment
FEV1	2.10 L	3.00 L	2.45 L	Shows moderate pre-test reduction with improvement after bronchodilator.
FVC	3.40 L	4.10 L	3.65 L	Helps interpret ratio and possible restriction.
FEV1/FVC	61.8%	LLN 70%	67.1%	Low ratio suggests airflow obstruction.
PEF	5.90	7.20	6.50	Useful for peak expiratory flow trend review.
FEF25-75	1.55	2.85	1.95	Optional supportive index for mid-flow change.
TLC	5.20 L	6.00 L	—	Low TLC supports restrictive physiology.

Formula Used

FEV1/FVC ratio (%) = (FEV1 ÷ FVC) × 100

Percent predicted = (Measured ÷ Predicted) × 100

Absolute bronchodilator change = Post − Pre

Percent change from baseline = ((Post − Pre) ÷ Pre) × 100

Percent change from predicted = ((Post − Pre) ÷ Predicted) × 100

Repeatability check compares the best and second-best values against the chosen tolerance.

Pattern logic uses ratio first, then low FVC or TLC to suggest obstruction, restriction, or mixed physiology.

Severity uses entered z-scores when available, otherwise FEV1 percent predicted fallback bands.

How to Use This Calculator

Enter the largest acceptable pre-bronchodilator FEV1 and FVC values.

Add predicted values from your chosen reference set.

Enter LLN values if your laboratory report provides them.

Optionally add TLC, RV, and z-scores for deeper interpretation.

Enter post-bronchodilator values to assess reversibility.

Add second-best FEV1 and FVC to review repeatability.

Submit the form to display the result block above the form.

Use the CSV and PDF buttons to export the summary.

FAQs

1) What does the FEV1/FVC ratio show?

A low ratio suggests airflow obstruction. The calculator compares the entered ratio with LLN when available, or a fixed fallback threshold when LLN is missing.

2) Why are predicted values important?

Predicted values convert raw liters into percent predicted. That helps adjust interpretation for body size and reference expectations rather than relying on liters alone.

3) What is LLN in pulmonary testing?

LLN means lower limit of normal. It marks the lower boundary of expected values in healthy people using a validated reference equation.

4) Why can FVC be low in obstruction?

Low FVC does not always mean restriction. In obstruction, air trapping and incomplete emptying can reduce FVC, which is why TLC helps clarify the pattern.

5) What is bronchodilator responsiveness?

It measures improvement after a bronchodilator. This file reports both a current percent-predicted rule and an older percent-plus-volume rule for side-by-side review.

6) Can this calculator diagnose asthma or COPD?

No. It organizes numeric interpretation only. Diagnosis still depends on symptoms, history, exposure profile, imaging, and formal clinician review.

7) Why does TLC matter for suspected restriction?

TLC helps confirm true volume loss. A normal ratio with low FVC can look restrictive, but TLC is more helpful for distinguishing restriction from nonspecific low-volume patterns.

8) Why check repeatability?

Repeatability shows whether the best efforts were close enough together. Better repeatability usually increases confidence that the reported spirometry values reflect true performance.