The Full Form of LLN is Lower limit of normal.

In normal spirometry, FVC, FEV₁, and FEV₁ -to-FVC ratio are above the lower limit of normal. The lower limit of normal is defined as the result of the mean predicted value (based on the patient’s sex, age, and height) minus 1.64 times the standard error of the estimate from the population study on which the reference equation is based. If the lower limit of normal is not available, the FVC and FEV₁ should be greater than or equal to 80% of predicted, and the FEV₁ -to-FVC ratio should be no more than 8-9 absolute percentage points below the predicted ratio. The ATS has recommended the use of lower limits of normal instead of the 80% of predicted for setting the threshold that defines abnormal test results.