Metrology in medicine

1
Metrology in medicine

• Martin Turner Senior Research Associate
  Douglas Joseph Fellow, Dept of
  Anaesthetics, University of Sydney
• Peter C Kam Prof Head of Department, Dept
  of Anaesthetics, UNSW.
• A Barry Baker Prof Head of Department, Dept
  of Anaesthetics, University of Sydney.
• Acknowledgements RB Frenkel, NML
• AJ Coleman, Guys St Thomas Hospital, UK

2
Measurements in medicine

• Aid diagnosis
• absolute values
• Guide treatment
• absolute values
• relative values or changes/trends(e.g. peak flow
  in asthma)

3
Old instruments vs modern

• Early medical instruments
• Simple, mechanical
• Faults were self-evident

• Modern instruments
• Black (coloured) box
• Software-based
• Digital display
• Faults lack of calibrationnot readily evident

4
Characteristics of medical measurements

• High variability(both intra and
  interindividual)
• Often indirect(e.g. cardiac output)
• Results often depend strongly on measurement
  technique
• Often uncertainty ? 10 20 is acceptable

5
Risk

• Medical measurements are made on sick people
• most diagnostic therapeutic procedures have
  associated risks
• a degree of risk associated with measurements is
  acceptable

6
Risk and variability

• Clinicians are familiar with the combination of
  variability risk
• Many clinicians
• assume variability due to lack of calibration is
  swamped by biological variability
• do not see a need for traceable calibration of
  medical instruments
• Many medical instruments are not traceably
  calibrated

7
Mitigating factors

• Diagnoses rarely depend on a single measurement
• Good clinicians
• know what results to expect
• disregard unusual measurements in the face of
  conflicting clinical evidence

8
Laws, regulationsstandards accreditation
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Therapeutic Goods Authority

• Falls under the Federal Dept of Health and Aging
• New medical devices must be licensed by the TGA
• Test equipment used to assess new equipment is
  required to be traceably calibrated
• Maintenance of existing medical equipment?

10
Standards

• AS/NZS 35511996. Technical management programs
  for medical devices.
• TEST EQUIPMENT Test equipment used shall be
  regularly calibrated to achieve traceability of
  measurement.
• AS/NZS 3551 is not enforced
• AS/NZS 3551 is ignored in some hospitals

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Accreditation

• The Australian Council on Healthcare Standards
  (ACHS) accredits Hospitals
• metrology issues are not prominent
• NATA accredits Pathology Biochem labs (Medical
  Testing Labs)
• Some professional bodies accredit their labs
  practitioners
• Quality control of other measurements appears to
  be at the discretion of individual clinicians,
  groups or hospitals

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Evidence-based medicine

• EBM is
• The conscientious, explicit and judicious use of
  current best evidence in making decisions about
  the care of individual patientsBMJ 1996
  312(7023)71-2.
• Presently there is no evidence that uncalibrated
  medical instruments cause adverse outcomes
  (Wilson et al. Med J Aust 1995163458. Med J
  Aust 1999 170411)
• Is traceable calibration likely to improve
  medical outcomes?

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Case study Blood pressure

• Most commonly measured physiological variable
• Diagnosis of hypertension depends entirely on BP
  measurements
• 29 of Australians ?25 yrs have hypertension (NHF)

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• Hypertension is an independent risk factor for
  coronary heart disease and cardiovascular
  disease.
• Cardiovascular diseases caused 40 of all deaths
  in Australia in 1998 (NHF).

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What is the effect of measurement error on the
diagnosis of hypertension?

• What is the range of measurement error?
• What is the sensitivity to those errors?
• AS EN 1060.1 2002 Non-invasive sphygmomanometers
  General requirements requires error lt 3 mm Hg

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UK Sphyg survey (J Hum Hyperten 15587 2001)
949 Hg sphygs 513 aneroid sphygs error gt 3 mm
Hg 16 error gt 5 mm Hg 9.3 error gt
10 mm Hg 3.7 1 in 54 UK
GP practices had sphygs calibrated
regularly
17
Distribution of diastolic BP
Source Joffres MR et al. Can Med Assoc J
146(11)1997 - 2005 1992 Data acquired 1986-1990
N 20582 data include treated subjects (10 of
total).
18
2
10
diastolic
systolic
1
10
subjects with pressure gt x
0
10
80
100
120
140
160

Blood pressure (mm Hg)
19

• Prob. that patient has true BP gt x p(x)
• Classification threshold xo
• Measurement error ?x
• Probability of classifying a patient
  hypertensive p(xo?x)
• Change in hypertensives

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0
-10
-20
-30
Systolic
hypertensive patients missed ()
-40
95 mm Hg
-50
-60
-70
Diastolic
85 mm Hgthreshold
-80
90 mm Hg
-90
-10
-8
-6
-4
-2
0
measurement error (mm Hg)
21
95 mm Hgthreshold
90 mm Hg
Diastolic
85 mm Hg
Systolic
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Effects of systematic error in BPat xo 95 mm Hg

• 3 mm Hg of 9 patients called
  hypertensive 4 are not
• 3 mm Hg half of true hypertensives
  missed
• 5 mm Hg of 13 patients called
  hypertensive 8 are not
• 5 mm Hg 2/3 of hypertensives missed

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Further work

• Determine the performance of sphygs in Australian
  GP practices
• Analyse the distribution of BP in Australia
• Estimate
• the number of normotensive people treated for
  hypertension
• the number of untreated hypertensives
• the number of cardiovascular deaths due to
  inadequate sphygmomanometer calibration

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Other measurements

• Temperature
• IR thermometers
• bias up to 2.3C
• 1.5C scatter
• Spirometry
• Electronic spirometers
• Type tested (inadequately?)
• In use calibration is inadequate

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Conclusions

• These examples may represent the tip of the
  medical metrology iceberg in Australia
• Inadequately calibrated medical instruments are
  an unrecognised cause of preventable medical
  errors
• Traceable calibration of medical measurement
  systems would
• Improve the quality of healthcare
• Reduce long term healthcare costs

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End
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Extras
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Temperature

• Tympanic IR thermometers are replacing
  Hg-in-glass thermometers
• Imamura et al. (Tokyo 1998) compared four
  different IR thermometers with tympanic
  thermocouples in vivo.

29
Imamura et al. Acta Anaesthesiol Scand 421222-6
1998.
30
Imamura et al. Acta Anaesthesiol Scand 421222-6
1998.
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Spirometry

• Commonly used for diagnosis treatment of asthma
  and COPD
• 2.2 million asthmatics in Australia
• 397 asthma deaths in Australia in 2002

32
Hankinson Crapo, Am J Respir Crit Care Med
1995 152696
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• ATS standard curves
• Forced vital capacity (FVC)
• FEV1
• Peak flow

1
7
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Spirometer calibration

• At manufacture Type-tested against 26 ATS curves
  using servo-driven syringe

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• In use
• volume checks only
• hand-driven 3 litre syringe

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• Servo-driven syringe has no correction for gas
  compression
• Many hand-driven syringes are not regularly
  calibrated
• Forced expiratory flow has high harmonic content,
  so static calibration (0 Hz only) is completely
  inadequate.

36
Imamura et al. Acta Anaesthesiol Scand 421222-6
1998.
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(No Transcript)
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• Professional bodies
• accredit some labs(Thoracic Society of Australia
  New Zealand accredits lung function labs)
• certify technicians in some fields(Australian
  NZ Soc of Respiratory Science certifies
  Respiratory Function Scientists)
• produce guidelines

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BP measurement error

• 2001 UK GP sphygmomanometers
• 3.7 had error gt 10 mm Hg
• 9.3 had error gt 5 mm Hg
• 16 had error gt 3 mm Hg
• 1 in 54 UK GP practices had sphygs regularly
  calibrated
• 1995/9 Australian sphygs (Newcastle)
• Sphyg maintenance poor
• J Qual Clin Pract. 19951517-22, 19991995-8