Service manual
IQmark Digital Spirometer Operation Manual
68
Lower Limit of Normal (LLN)
The LLN for FVC, FEV
1
and FEV
1
/FVC is calculated as follows:
CIValueLLN
pred %95
The Value
pred
is the predicted value provided by the reference equations. The CI
95%
is the
95% confidence interval for the predicted value. If the CI
95%
is not reported for the
predicted value, and the Standard Error of the Estimate (SEE) is, then the CI
95%
shall be
calculated as follows:
SEECI
x
645.1
%95
If the SEE is not reported and the SD% (Standard Deviation of the error) is, then the
software shall calculate the LLN as follows:
100
%
1
2
SD
ValueLLN
x
pred
F. Interpretation – NHANES III
The 3rd National Health And Nutrition Examination Survey (NHANES III) created a
number of medical recommendations including some directed at Spirometry. A new set
of predicted equations as well as new calculations for LLN (lower limit of normal) were
developed from data collected and we have labeled these equations as NHANES III.
The National Lung Health Education Program (NLHEP www.nlhep.org) published an www.nlhep.org) published an
Interpretation table using NHANES III. We have labeled this interpretation logic as
NHANES III (or by selecting PCP Mode as the Configuration Profile, see Configuring
The Spirometer). It determines results as follows:
The software displays a statement that reads "Normal Spirometry" if the FEV
1
and the
FEV
1
/FEV
6
ratio are within the normal range (i.e. above the LLN).
If the FEV
1
/FEV
6
ratio and the FEV
1
are below the LLN, the software will display one of
the following statements regarding obstruction.
Statement Criteria
Mild obstruction. FEV
1
>= 60% Pred. FEV
1
Moderate obstruction. 40% Pred. FEV
1
<= FEV
1
< 60% Pred. FEV
1
Severe obstruction. FEV
1
< 40% Pred. FEV
1
In addition, if the FEV
1
/FEV
6
ratio is above the LLN but the FEV
6
measurement is below
the LLN, the software will output the following statement:
"Low vital capacity, perhaps due to restriction of lung volumes"
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