Getting PFTs Right When the Numbers Don't Make Sense
Pulmonary function testing sounds straightforward in theory. You hook someone up to a spirometer, they blow into a tube, and the machine spits out numbers. In practice, it's a lot of work to figure out what those numbers actually mean for the person sitting in front of you. I've spent years doing this, and the hardest part isn't the calculation. It's knowing when the data is garbage and how to recover from it. The first thing you do is quality check. Every test needs to meet the ATS/ERS criteria before you even think about interpretation. Good effort, acceptable start of test, minimal coughing in the first two seconds, a proper back-extrapolated volume, and a plateau on the volume-time curve that holds for at least one second. If the test doesn't meet these standards, you don't interpret it. You repeat it. I've seen reports come out of clinics with obviously submaximal efforts mislabeled as "mild obstruction" because nobody bothered to check the flow-volume loop for a proper expiratory effort. The loop will tell you if the patient didn't push hard enough. Look at the peak expiratory flow rate. If it rises gradually instead of spiking sharply at the start, the patient hesitated on initiation. That invalidates the forced vital capacity and the FEV1. You discard the test and ask for another one. This happened to me last year with a COPD patient whose spirometry appeared normal until I checked the loop shape. He'd been holding back out of habit. After a proper coaching run, his FEV1 dropped 40% from the first attempt. The first reading would have sent him home with no treatment.
Interpretation Of Pulmonary Function Tests A Practical Guide
Once you have a acceptable quality test, the interpretation follows a set sequence. You check obstruction first. The FEV1/FVC ratio is the gatekeeper. If it's below the lower limit of normal — not the fixed 0.70 cutoff that's still too common in some labs — there's an obstructive defect. The LLN accounts for age, height, sex, and ethnicity, which matters more than most people realize. A 70-year-old white male with an FEV1/FVC of 0.68 might actually be normal by LLN standards, while a 45-year-old woman with the same ratio is clearly obstructed. Using a fixed 0.70 cutoff systematically misses obstruction in older patients and overdiagnoses it in younger ones. Then you assess severity using the FEV1 percent predicted. Mild is 70% or above, moderate is 60 to 69%, severe is 50 to 59%, and very severe drops below 50%. But severity alone doesn't tell the whole story. You also need the FVC. If the FVC is disproportionately reduced compared to the FEV1, you're looking at possible air trapping or a concurrent restrictive process. This is where I see the most mistakes. People stop at the obstruction diagnosis and miss the restriction hiding underneath. To confirm restriction, you need lung volumes. Spirometry alone can't diagnose it reliably. TLC below the LLN is the definitive criterion. If you don't have plethysmography or gas dilution available and the spirometry shows a reduced FVC with a normal or elevated FEV1/FVC, you should order lung volumes before calling it restriction. A reduced FVC could just be poor effort, air trapping, or an obstructive pattern masquerading as restriction. I had a patient recently with what looked like restriction on spirometry — low FVC, normal ratio. Lung volumes showed an elevated RV and normal TLC. It was pure emphysema with air trapping, not restriction. The misinterpretation on the initial read would have led us down the wrong diagnostic pathway entirely.
The flow-volume loop is where the real patterns emerge. Concentric narrowing suggests diffuse airway disease like asthma or COPD. A flat inspiratory limb points to variable intrathoracic obstruction — think tracheal stenosis or a mass compressing the airway from the outside. A flat expiratory limb alone also indicates intrathoracic fixed obstruction. Two-trap patterns — flattening on both limbs — suggest a fixed upper airway obstruction like subglottic stenosis or tracheomalacia. I encountered a case a few years back where the spirometry numbers were barely abnormal but the flow-volume loop showed a dramatic extrathoracic pattern. An awake laryngoscopy revealed a vocal cord paralysis that the lab technician had completely missed. The numeric values barely moved off the chart, but the loop told the whole story. Diffusion capacity adds another dimension. DLCO measures how well gas crosses the alveolar-capillary membrane. Low DLCO with normal lung volumes points to an interstitial process or pulmonary vascular disease. Low DLCO with increased lung volumes is classic emphysema — you've lost capillary bed surface area even though the lungs are hyperinflated. Here's the counterintuitive part that trips people up: asthma can present with a normal or even elevated DLCO due to increased pulmonary capillary blood volume from chronic inflammation. Don't let a normal DLCO rule out significant lung disease in an asthmatic. I've had pulmonologists pause on their diagnosis when they saw a normal diffusion capacity in patients who clearly had obstructive symptoms. The DLCO was the red herring. Bronchodilator response interpretation has its own pitfalls. A positive response is defined as an increase of more than 12% and 200 mL in either FEV1 or FVC from baseline. The problem is that percentage changes are meaningless without the absolute volume change, and vice versa. A patient with an FEV1 of 1.0 L who improves by 150 mL hits 15% but falls short on the absolute criterion. Another patient with an FEV1 of 2.5 L who improves by 300 mL is clearly significant. Both changes matter. Some patients show a large percentage improvement from a low baseline that's clinically irrelevant, while others with moderate obstruction show a modest percentage change that represents genuine clinical improvement. Always report both the percentage and the absolute change.
Get the Full Details
Exercise spirometry is increasingly relevant but poorly understood. Some patients have normal resting spirometry but demonstrate exercise-induced bronchoconstriction — a drop in FEV1 of 10% or more after exercise. The traditional methacholine challenge is being supplemented by this approach in sports medicine and occupational health. The practical issue is standardization. You need controlled exercise intensity, usually targeting 85 to 90% of predicted heart rate, for at least six minutes. The testing environment and cooling conditions matter enormously. Cold dry air triggers responses that warm humid air won't. I had a runner referred for unexplained exercise tolerance decline. Resting PFTs were normal. Standard bronchoprovocation was negative. Exercise spirometry at 88% predicted maximum heart rate in cool air showed a 14% FEV1 drop. The diagnosis was exercise-induced bronchoconstriction that only manifested under those specific conditions. This is exactly the kind of edge case that sits between diagnosis and missing the problem entirely.
When The Interpretation Falls Apart
No guide covers everything, and some patient populations consistently break standard interpretation algorithms. Obesity is one. BMI above 30 compresses the diaphragm and reduces chest wall compliance, producing a restrictive pattern on spirometry even when lung volumes are normal. I've reviewed reports where obese patients were labeled as having interstitial lung disease based on spirometry alone, and the HRCT came back completely clear. Always get body surface area-corrected predictions or actual lung volumes in this population. Cerebrovascular accidents and neuromuscular disease create another category of difficulty. These patients often can't generate a proper forced expiration despite having normal lung mechanics. The maximum voluntary ventilation will be low, the peak flows are weak, and the flow-volume loop looks like a sloping hill instead of a sharp spike. Calling this obstruction is wrong. It's a weak pump problem, not an airway problem. The distinction matters because the management is completely different. Respiratory muscle training and noninvasive ventilation are the considerations here, not bronchodilators. Pediatric interpretation has its own separate set of standards. Children under 12 or under 45 kg generally can't produce reproducible forced maneuvers reliably enough for full ATS interpretation. The pediatric guidelines allow for different acceptance criteria, but most adult protocols simply don't apply. I once reviewed a report on an 8-year-old where the lab had applied adult LLN values and declared moderate obstruction. The child's values were perfectly normal for age and height. Using the correct pediatric reference equations — preferably the GLI-2012 pediatric set — would have prevented that error.
The most frustrating limitation I encounter is with single-breath DLCO in patients with anemia or polycythemia. Hemoglobin directly affects CO uptake. The standard correction factor adjusts for hemoglobin, but if you don't have a recent hemoglobin value on hand, the DLCO result is essentially a guess. I always check the complete blood count before signing off on a diffusion capacity report. A hemoglobin of 8 g/dL will artificially lower DLCO by roughly 15%, which could flip a borderline result into an abnormal classification. Conversely, a hematocrit of 55% will falsely elevate it. This isn't theoretical. I corrected three abnormal DLCO interpretations in a single month last year after pulling the lab data and finding uncorrected anemias. Reference equation selection is another area where small decisions create large downstream errors. The GLI-2012 equations are now the standard for spirometry and lung volumes in most of the world, replacing the older NHANES III and European Coal and Steel Community references. They cover a much wider range of ages and ethnicities with continuous coefficients rather than categorical adjustments. Some labs in the United States still default to NHANES III for African American patients, which the GLI has shown to be less accurate across the full age spectrum. The difference is usually small — 3 to 5% predicted on average — but in borderline cases, it changes the diagnosis from normal to abnormal or vice versa. Check which equations your lab uses and whether they've transitioned to GLI. Here's something most guides won't tell you: serial comparison matters more than any single value. A patient's FEV1 dropping from 85% predicted to 70% predicted is clinically significant even though both values fall within the "normal" range by LLN standards. Trend data over multiple visits carries more diagnostic weight than an isolated measurement. Keep good records of every test. The trajectory tells you more than the destination.
If you're looking for a structured reference to work through, Interpretation Of Pulmonary Function Tests A Practical Guide format tends to work best when it mirrors this sequence: quality check, obstruction screen, restriction confirmation, flow-volume pattern recognition, diffusion capacity context, and clinical correlation. Skip any step and you'll miss something. The whole point of having a systematic approach is that the pattern you're looking for might be hidden in the step you're tempted to shortcut. I should also mention that online calculators and automated interpretation tools are useful but unreliable on their own. Many generate text reports that sound authoritative but contain factual errors or miss clinically relevant findings. I've seen automated reports describe normal spirometry for a patient whose flow-volume loop clearly showed a flat inspiratory limb indicating upper airway obstruction. The algorithm had evaluated the numeric values correctly but completely ignored the graphical data. Always review the raw traces before accepting any machine-generated interpretation. The computer does the arithmetic. You do the medicine.
