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Interpretative strategies

Spirometric test results should be interpreted in conjunction with predicted values and their lower limits, clinical data, and when available the response to bronchodilator drugs.
There are many publications on predicted values. The ‘lower limit of normal’ is commonly derived in either of three ways:

  1. Predicted value minus 1.645 the standard deviation of the predicted value. This is a procedure to estimate the lower 5th percentile. It is based on the assumption that the scatter around the predicted value is normally distributed. In practice this condition is often not fully met.
  2. From the 5th percentile, the index adopted in SpirXpert software. Unfortunately in most publications the 5th percentile is not mentioned, so that option 1 is then the best approximation.
  3. Use 80% of the predicted value als the ‘lower limit of normal’. This is scientifically unfounded (ref. 1), but nevertheless often done. This is not used in SpirXpert software.

It matters which set of reference values is adopted, and how the ‘lower limit of normal’ is determined, in particular in the case of borderline values. This can be shown by the following case of a Caucasian male, standing height 1.78 m, age 55 yr, observed FEV1 3.06 L. Predicted values and lower limits according to some published equations are as follows:

Author predicted
(liter)		 predicted -1,645·RSD
(liter)		 predicted LLN
(liter)		 80% predicted
(liter)		 observed
3.06 liter
LLN 80%pred
Berglund 4.11 3.21   3.29 too low too low
Brändli 3.86   3.07 3.09   too low
Crapo 3.84   3.00 3.07   too low
ECCS/ERS 3.57 2.73 - 2.86    
ECCS/ERS adjusted 3.85   2.98 3.08   too low
Falaschetti 3.73   2.94 2.98    
Forche 3.93 3.06   3.14   too low
Gore/Crockett 3.81 3.01   3.04    
Hedenström 3.96 2.78 - 3.17   too low
Knudson 3.72   2.87 2.98    
Langhammer 3.95   3.24 3.16 too low too low
Morris 3.63 2.70 - 2.90    
NHANES III 3.78   2.99 3.02    
Pereira 3.73 2.94   2.98    
Viljanen 3.98 2.96   3.18   too low
Roca 3.88 3.15   3.10 too low too low

As shown in the table, both the sensitivity and the specificity of spirometric test results is influenced by the selection of reference values as well as by the strategy adopted to compute the ‘lower limit of normal’:

  1. There are appreciable differences between predicted values available in SpirXpert and their associated ‘lower limits of normal’. On that account the FEV1 of 3.06 L is within the ‘normal range’ according to most prediction equations when using either the 'lower limit of normal' is specified by the author, or approximating it from the predicted mean minus 1.64·RSD. Application of the 80% rule which for obscure reasons is still used leads to a considerable bias and potentially to overdiagnosis and overtreatment.
  2. Please note that in younger and in elderly subjects, and in taller subjects, the 80% rule for establishing the ‘lower limit of normal’ leads to greater errors of judgement.

See also:
Reference values for adults
SpirXpert software

Ref. 1 - Do not use per cent predicted
1 Sobol BJ. Assessment of ventilatory abnormality in the asymptomatic subject: an exercise in futility. Thorax 1966; 21: 445-449.
2 Oldham PD. Percent of predicted as the limit of normal in pulmonary function testing: a statistically valid approach. Thorax 1979; 34: 569.
3 Miller A. Prediction equations and ‘normal values’. In: Miller A, ed. Pulmonary function tests in clinical and occupational lung disease. New York, Grune & Stratton, 1986; 197-213
4 Miller MR, Pincock AC. Predicted values: how should we use them? Thorax 1988; 43: 265-267.
5 Quanjer PhH. Predicted values: how should we use them (letter). Thorax 1988; 43: 663-664.
6 ATS Statement. Lung function testing: selection of reference values and interpretative strategies. Am Rev Respir Dis 1991; 144: 1202-1218.
7 Quanjer PhH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. Lung volumes and forced ventilatory flows. Eur Respir J 1993; 6 suppl. 16: 5-40.
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