Measurements of Pulmonary Function Instrument Accuracy - ndd.ch

Table 3—CV and Measurement Errors in Mean
Absolute Accuracy for DLCO End Points
Instrument
CV for Dlco End Points
(Errors*)
Dlco VI VA
Collins (n � 180) 2.52 (6) 0.79 0.37
Morgan (n � 180) 6.06 (90) 4.24 1.94
SensorMedics (n � 180) 2.30 (5) 1.96 1.83
Jaeger (n � 180) 4.04 (70) 3.13 1.51
MedicalGraphics (n � 180) 6.73 (141) 3.99 3.78
*Dlco end point errors are defined in the �Materials and Methods�
section.
tatively, recognizing that only one instrument model
was studied from each of the five manufacturers. We
cannot state that our estimates of accuracy and
reproducibility are representative of the models as a
whole.
It is particularly important to understand whether
the performance characteristics of instruments are
changing over time. For Dlco, a cumulative change
in the percentage of accuracy as high as 20.9% was
observed in one instrument (Table 4). This would
translate into a change in Dlco of about 4 mL/
min/mm Hg (1.35 mmol/min/kPa) in a patient with
an initial Dlco of 20 mL/min/mm Hg (6.70 mmol/
min/kPa), due only to changes in instrument performance
over time. Several potential problems can
cause errors or drift in Dlco measurements. They
include gas analyzer failure, volume measurement
error, improper technique, and gas sample-conditioning
problems. Most systems calibrate gas analyzers
just prior to a Dlco test, which should reduce
errors due to small analyzer drift. Changes in analyzers
from a linear to a nonlinear output might
account for the long-term drifts observed. The specific
causes of accuracy drift should be investigated
in future studies. In the meantime, our results
strongly suggest that regular monitoring of known
values should be performed in clinical and research
PFT laboratories, through the use of biological control
or simulator testing.
This study has implications for pulmonary function
testing in both clinical and research settings. Accuracy
and reproducibility are considerations when
purchasing an instrument for laboratory use. In
clinical testing, instruments with lower reproducibility
may require more trials to meet ATS/ERS testing
standards, thus requiring more technician time and
supplies than instruments with better reproducibility.
Research studies to detect differences in pulmonary
function between patient cohorts can be adversely
affected if a more variable instrument is used
because the level of differences that can be detected
is a function of the number of subjects and the
reproducibility of the tests.
Our study has aspects that may limit its interpretation.
Only one instrument from each manufacturer
was tested, which may not be representative of other
instruments produced by a given manufacturer. Artifacts
due to gas compression 15 or wave action 16
could have influenced the reported accuracy of the
PEF values for some of the 24 ATS waveforms. We
did not test human volunteers or patients. They
would be expected to reduce the reproducibility
observed. The biological components associated with
the reproducibility of PFT results in human subjects
have been discussed in a companion article. 17 In
summary, this study provides a quantitative assessment
of the accuracy and reproducibility of spirom-
Table 4—Change in Percentage of Accuracy in FEV 1 and DLCO Between Time Points*
Instrument � Day 0–30, % � Day 30–60, % � Day 60–90, % Overall, %
Collins
FEV 1 �0.01 �0.43‡ 0.46† 0.02
Dlco �1.50† �0.70 �1.70† �3.9†
Morgan
FEV 1 �0.53† �0.22† 0.62† �0.13
Dlco 4.20 8.90† 4.90 18.0†
SensorMedics
FEV 1 1.53† �0.49† �0.19 0.85†
Dlco 0.00 1.40 �0.10 1.30
Jaeger
FEV 1 �0.55 1.52† �2.72† �1.75†
Dlco 1.90 2.50 16.50† 20.9†
MedicalGraphics
FEV 1 �0.83† �0.55† �0.08 �1.46†
Dlco 0.90 �3.40 �14.70† �17.2†
*Percent accuracy � (�observed � target�/target) � 100; Overall � sum of time point differences in accuracy.
†p � 0.008.
394 Original Research
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