Step 2: Determine If the FVC Is Low The physician must determine if the FVC is less than the LLN for adults or less than 80% of predicted for those five to 18 years of age, indicating a restrictive pattern. The LLN can be determined using the calculator at http://hank consulting.com/RefCal.html. A restrictive pattern can indicate restrictive lung disease, a mixed pattern (if a patient has an obstructive defect and a restrictive pattern), or pure obstructive lung disease with air trapping. Table 2 summarizes the first two steps of PFT interpretation.