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It’s a Gas!
Issues in the Blood
Gas Laboratory
D. Robert Dufour, MD, FCAP
Consultant Pathologist
VA Medical Center, Washington DC
2
Learning Objectives
After participating in this session, you will be
able to:
1) Develop valid means for comparing different
blood gas analyzers and for confirming
analytical measurement range.
2) Review QC results to detect important
differences between instruments at an early
time.
3) Institute procedures to detect and prevent
preanalytical variation in blood gas results.
3
Requirements for the
Blood Gas Laboratory
• There are a limited number of questions that
are specific to the blood gas section of the
laboratory
• There are a number of questions in both the
Lab General and Chemistry Checklists that
may have special considerations in the blood
gas lab
• This presentation will attempt to address
issues related to both
4
Sample Collection
• Blood gases are typically collected from
arteries; two questions relate to training
for arterial puncture (CHM.33800) and
procedure itself (CHM.33900)
• Question requires testing for adequate
collateral circulation (such as Allen test),
when appropriate; only applies if lab
staff draw samples
5
Collateral Circulation
• Checklist does not specify a particular
technique for testing collateral circulation,
nor patients in whom it must be done
• Does say that consensus should be
established between clinicians, laboratory
on patients who should be assessed
6
Collateral Circulation
• Can most easily be established by
hospital or lab policy, jointly approved
by medical staff
• Best to document in lab report or
medical record that test was performed
• Also required to specify site of collection
(only mentioned in note)
7
Collateral Circulation
• Issue related to anatomy; in about half
of individuals, palmar arterial arches
incomplete, and in about 4%, loss of
radial artery could compromise blood
supply to the hand
• More of a concern if ulnar blood flow
also compromised, as can occur with
severe atherosclerosis
8
Collateral Circulation
• Henry’s textbook says modified Allen
test should be performed in all patients
prior to radial artery puncture
• Modified Allen test – occlude both ulnar
radial arteries, have clench fist, release
ulnar, open fist: assure adequate flow
by return of red color; can repeat with
radial artery decompression
9
Collateral Circulation
• “Automated” version – more reliable in
hypotensive or unresponsive; use pulse
oximeter on index finger, release ulnar,
should see pulse detected within 15 sec
• Evaluation of oximeter reading itself produces
false positive results (while Allen test has
false negative results), so use of the pulse
sensing portion of pulse oximeters may be
most reliable
10
Ambient Air
• Final sample question requires system
to prevent ambient air contamination
• Air bubbles cause equilibration between
sample and room air.
• This always cause a fall in PCO2 and pH.
• Effect on PO2 depends on treatment;
usually ↑, but may be ↓ if patient on
high FIO2
11
Ambient Air
• Most common cause: air bubbles; effect
of bubbles increased if many small
bubbles, by delay in analysis, or
transported by pneumatic tube
• Policies/procedures should include
sample examination and documentation
of likely erroneous results
12
Instrument Questions
• Only 6 specific questions (CHM.34100-
34600) on blood gas instruments; these
apply in a lab doing blood gases and
other chemistry tests with same director
(even if in separate location(s))
• If physically separate lab, separate
director, then full Chemistry Checklist
must be evaluated for blood gas lab
13
Instrument Questions
• CHM.34100 – Straightforward function
checks – assure you have
documentation of maintenance, etc.
• CHM.34200 – Calibrators meet
manufacturer’s specifications or NIST
SRM – easily satisfied if use
manufacturer’s calibrators
14
Instrument Questions
• CHM.34300 – Calibration and cal
verification – Must follow manufacturers
recommended schedule if used
regularly; for self-calibrating
instruments, not an issue
• If used infrequently, must be calibrated
each time of use
15
Instrument Questions
• Question also mentions barometric
pressure being considered, if
appropriate; follow manufacturer’s
recommendations in this case
• Note requires some procedure for
verifying reliability of calibration; two
expert reviewers have suggested
deleting or modifying this note
16
Instrument Questions
• CHM.34400, 34500 – At least 1 level of
control every 8 hours; at least 2 levels
of control (high, low) every day
• Defined by 8 hours, day of use for
patient testing
• Can use tonometered material or liquid
commercial controls
17
Instrument Questions
• Except for assayed controls, each lab
must establish own control range based
on their measurements, and not use the
ranges provided (CHM.14000)
• Use of too wide ranges can lead to
failure to detect bias and to higher risk
of PT failure
18
Instrument Questions
• CHM.34560 – Requires one control
sample each time patients are tested
unless auto-calibrated every 30 min
• This would likely apply to instruments
that are used infrequently during the
day, not to an instrument that is used
multiple times during a shift
19
Proficiency Testing
• CHM.10200 requires PT to be
integrated with regular workload; note
suggests best rotated among all those
who regularly perform testing
• Performance can be used as part of
competency testing for employees if
regularly rotated
20
Proficiency Testing
• If more than one instrument used, not
required to be tested on all instruments,
only primary instrument
• Important to pay careful attention to
instructions for handling PT material for
blood gases; results can be adversely
affected if directions not followed
21
Calibration Verification
• CHM.13400 requires criteria for
accuracy of calibration and/or for
calibration verification
• Since blood gas instruments calibrated
very frequently, calibration verification is
not required
22
Sample Acceptability
• CHM.11900 requires lab to have
specified criteria for sample
acceptability
• Note indicates that samples that are
unacceptable can still be reported, but
with laboratory noting that results may
be unreliable because of sample
condition
23
Sample Acceptability
• In blood gas laboratory, common
causes for sample unacceptability are:
• 1) Low volume (esp. with liquid heparin,
causes falsely high PO2)
• 2) Air bubbles (causes falsely high PO2,
falsely low PCO2)
• 3) Inadequate icing/delayed arrival
(causes falsely low pH)
24
Sample Acceptability
• CHM.12000 requires documentation of
action taken on unacceptable samples
• Can be either in patient records (as
result or notation), or in sample error
log/form
• Latter approach makes review easier for
QA purposes
25
Reagent Validation
• CHM.12900 requires that all new lots be
validated before being put into use
• Most easily performed by analyzing
patient samples with old, new lots (we
use same form as for method
comparison done every 6 months, and
same criteria for acceptability)
26
Reagent Validation
• Other materials (QC, for example) can
also be used
• Lab must have documentation that this
behaves similarly to patient samples
• Not as much an issue in blood gas
testing as in many other areas of lab
27
Verify Reportable Range
• Reportable range includes:
– Analytical measurement range (AMR) –
range of values that instrument can report
directly (less accurately called linearity)
– Clinically reportable range (CRR) – range
of values that can be reported with dilution
or concentration of samples; for blood gas
lab, not applicable
28
Analytical Measurement Range
• CHM.13600 requires determination of
AMR before method used, and
reverification at least every 6 months,
using at least 3 samples that span
entire measurement range
• Not required separately if at least 3
calibrators that span range used
29
Analytical Measurement Range
• For PCO2, PO2, difficult to get patient
samples that span measurement range,
and these are not stable on storage
• Various vendors (including CAP)
produce linearity materials that can be
used to verify AMR; in our lab, we use
CAP LN-13 survey for this purpose
30
Analytical Measurement Range
• For most recent survey, ranges covered
were:
– pH: 6.8-8.8, allowed difference ± 0.3%
– PCO2: 12-91.5, allowed difference ± 5%
– PO2: 37-445, allowed difference ± 8%
• Remember, values outside AMR must
be reported > or < limits
31
Method Comparison
• CHM.13800 requires that, if more than
one method used for measuring same
parameter, labs check against each
other at least 2x yearly for patient
results
• Requires development of criteria for
acceptable agreement
32
Method Comparison
• Note indicates patient samples are
preferable, but may not be possible for
unstable analytes if instruments located
at some distance (often an issue with
blood gas instruments/samples)
• If controls or reference material used,
lab must verify they perform similarly to
patient samples
33
Method Comparison
• Does not require that single set of
samples be used to compare all
instruments; may compare instrument 1
and 2 with one set, instrument 1 and 3
with another set
• Criteria should be based on patient care
needs and/or criteria needed to pass
proficiency testing
34
Method Comparison
• Be careful to not use “blanket” criteria
for whole laboratory; for example, in
recent inspection I performed,
acceptable agreement for all tests was
± 10%
• For pH, at target value of 7.40, this
would mean “acceptable” was between
6.66 and 8.14!
35
Method Comparison
• For CAP proficiency testing (based on
CLIA-88 regulations), acceptable
performance is within:
– pH: ± 0.04
– PCO2: ± 5 mm Hg or 8%, whichever is
lower
– PO2: ± 3 s.d.
36
Method Comparison
• Question and note do not specify actual
method to be used or number of
samples; in our lab, we perform this
analysis with 5 samples
• Best to use a range of values (both
normal and abnormal high and low) to
detect any biases
37
Method Comparison
• We use form, fill in test, ID of samples
used, results from different instruments,
and director review of acceptability
• We have a single sheet for each section
on allowable differences between
methods developed by director, based
on typical variation in patients (see Tietz
or Kaplan & Pesce for tables)
38
Reference Intervals
• CHM.15000 requires all results to be
reported with reference intervals
• With point of care instruments, printout
often does not have this information on
it, and is sometimes the only “report”
available to clinicians
• Lab must assure that some form with
reference intervals is attached to report
39
Critical Values
• CHM.15100 requires labs to establish criteria
that require immediate notification
• It is not required that all areas have such
critical values, but it is common for blood
gases to have critical values
• May want to have separate critical values by
age (for example, high PO2 dangerous in
infants, not so in children, adults)
40
Critical Values
• CHM.15200 requires documentation of
prompt notification of physician
• If blood gas lab is under separate
leadership (e.g., respiratory medicine),
important that similar approach used to
what is done in central lab
• Often, critical values are hospital, not
laboratory, policies
41
Carryover
• CHM.24400 requires labs to evaluate
automatic pipetting systems for carryover
• Since blood gas instruments often use
pipetting systems, needs to be done for
these instruments as well
• Protocol involves running high sample
several times, then repeat low sample
42
Carryover
• Degree of difference for low sample
(from when run previously) should be
within lab’s criteria for acceptable
difference between instruments
• Seldom an issue in blood gas lab
because of the limited range of results
obtained, but needs to be done once
43
Quality Management
• A number of questions on QM are found
in lab general section
• Remember that QM provisions apply to
blood gas laboratory, and some QA
monitor must be done if blood gas
section is separate from main laboratory
44
Summary
• If blood gas lab is part of main
laboratory, there are few specific
requirements
• If separate section with different director
(common situation), blood gas lab must
comply with full Chemistry, Lab
General, and Team Leader Checklist
requirements
45
Summary
• Important to note differences in how
some questions apply to blood gas labs
because of differences in type of
samples and their stability
• This session covered some of the most
common areas where these differences
are important
46
Assistance
http://www.cap.org
Email: accred@cap.org
800-323-4040, ext. 6065
chemdoctorbob@earthlink.net

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Cap blood gas

  • 1. It’s a Gas! Issues in the Blood Gas Laboratory D. Robert Dufour, MD, FCAP Consultant Pathologist VA Medical Center, Washington DC
  • 2. 2 Learning Objectives After participating in this session, you will be able to: 1) Develop valid means for comparing different blood gas analyzers and for confirming analytical measurement range. 2) Review QC results to detect important differences between instruments at an early time. 3) Institute procedures to detect and prevent preanalytical variation in blood gas results.
  • 3. 3 Requirements for the Blood Gas Laboratory • There are a limited number of questions that are specific to the blood gas section of the laboratory • There are a number of questions in both the Lab General and Chemistry Checklists that may have special considerations in the blood gas lab • This presentation will attempt to address issues related to both
  • 4. 4 Sample Collection • Blood gases are typically collected from arteries; two questions relate to training for arterial puncture (CHM.33800) and procedure itself (CHM.33900) • Question requires testing for adequate collateral circulation (such as Allen test), when appropriate; only applies if lab staff draw samples
  • 5. 5 Collateral Circulation • Checklist does not specify a particular technique for testing collateral circulation, nor patients in whom it must be done • Does say that consensus should be established between clinicians, laboratory on patients who should be assessed
  • 6. 6 Collateral Circulation • Can most easily be established by hospital or lab policy, jointly approved by medical staff • Best to document in lab report or medical record that test was performed • Also required to specify site of collection (only mentioned in note)
  • 7. 7 Collateral Circulation • Issue related to anatomy; in about half of individuals, palmar arterial arches incomplete, and in about 4%, loss of radial artery could compromise blood supply to the hand • More of a concern if ulnar blood flow also compromised, as can occur with severe atherosclerosis
  • 8. 8 Collateral Circulation • Henry’s textbook says modified Allen test should be performed in all patients prior to radial artery puncture • Modified Allen test – occlude both ulnar radial arteries, have clench fist, release ulnar, open fist: assure adequate flow by return of red color; can repeat with radial artery decompression
  • 9. 9 Collateral Circulation • “Automated” version – more reliable in hypotensive or unresponsive; use pulse oximeter on index finger, release ulnar, should see pulse detected within 15 sec • Evaluation of oximeter reading itself produces false positive results (while Allen test has false negative results), so use of the pulse sensing portion of pulse oximeters may be most reliable
  • 10. 10 Ambient Air • Final sample question requires system to prevent ambient air contamination • Air bubbles cause equilibration between sample and room air. • This always cause a fall in PCO2 and pH. • Effect on PO2 depends on treatment; usually ↑, but may be ↓ if patient on high FIO2
  • 11. 11 Ambient Air • Most common cause: air bubbles; effect of bubbles increased if many small bubbles, by delay in analysis, or transported by pneumatic tube • Policies/procedures should include sample examination and documentation of likely erroneous results
  • 12. 12 Instrument Questions • Only 6 specific questions (CHM.34100- 34600) on blood gas instruments; these apply in a lab doing blood gases and other chemistry tests with same director (even if in separate location(s)) • If physically separate lab, separate director, then full Chemistry Checklist must be evaluated for blood gas lab
  • 13. 13 Instrument Questions • CHM.34100 – Straightforward function checks – assure you have documentation of maintenance, etc. • CHM.34200 – Calibrators meet manufacturer’s specifications or NIST SRM – easily satisfied if use manufacturer’s calibrators
  • 14. 14 Instrument Questions • CHM.34300 – Calibration and cal verification – Must follow manufacturers recommended schedule if used regularly; for self-calibrating instruments, not an issue • If used infrequently, must be calibrated each time of use
  • 15. 15 Instrument Questions • Question also mentions barometric pressure being considered, if appropriate; follow manufacturer’s recommendations in this case • Note requires some procedure for verifying reliability of calibration; two expert reviewers have suggested deleting or modifying this note
  • 16. 16 Instrument Questions • CHM.34400, 34500 – At least 1 level of control every 8 hours; at least 2 levels of control (high, low) every day • Defined by 8 hours, day of use for patient testing • Can use tonometered material or liquid commercial controls
  • 17. 17 Instrument Questions • Except for assayed controls, each lab must establish own control range based on their measurements, and not use the ranges provided (CHM.14000) • Use of too wide ranges can lead to failure to detect bias and to higher risk of PT failure
  • 18. 18 Instrument Questions • CHM.34560 – Requires one control sample each time patients are tested unless auto-calibrated every 30 min • This would likely apply to instruments that are used infrequently during the day, not to an instrument that is used multiple times during a shift
  • 19. 19 Proficiency Testing • CHM.10200 requires PT to be integrated with regular workload; note suggests best rotated among all those who regularly perform testing • Performance can be used as part of competency testing for employees if regularly rotated
  • 20. 20 Proficiency Testing • If more than one instrument used, not required to be tested on all instruments, only primary instrument • Important to pay careful attention to instructions for handling PT material for blood gases; results can be adversely affected if directions not followed
  • 21. 21 Calibration Verification • CHM.13400 requires criteria for accuracy of calibration and/or for calibration verification • Since blood gas instruments calibrated very frequently, calibration verification is not required
  • 22. 22 Sample Acceptability • CHM.11900 requires lab to have specified criteria for sample acceptability • Note indicates that samples that are unacceptable can still be reported, but with laboratory noting that results may be unreliable because of sample condition
  • 23. 23 Sample Acceptability • In blood gas laboratory, common causes for sample unacceptability are: • 1) Low volume (esp. with liquid heparin, causes falsely high PO2) • 2) Air bubbles (causes falsely high PO2, falsely low PCO2) • 3) Inadequate icing/delayed arrival (causes falsely low pH)
  • 24. 24 Sample Acceptability • CHM.12000 requires documentation of action taken on unacceptable samples • Can be either in patient records (as result or notation), or in sample error log/form • Latter approach makes review easier for QA purposes
  • 25. 25 Reagent Validation • CHM.12900 requires that all new lots be validated before being put into use • Most easily performed by analyzing patient samples with old, new lots (we use same form as for method comparison done every 6 months, and same criteria for acceptability)
  • 26. 26 Reagent Validation • Other materials (QC, for example) can also be used • Lab must have documentation that this behaves similarly to patient samples • Not as much an issue in blood gas testing as in many other areas of lab
  • 27. 27 Verify Reportable Range • Reportable range includes: – Analytical measurement range (AMR) – range of values that instrument can report directly (less accurately called linearity) – Clinically reportable range (CRR) – range of values that can be reported with dilution or concentration of samples; for blood gas lab, not applicable
  • 28. 28 Analytical Measurement Range • CHM.13600 requires determination of AMR before method used, and reverification at least every 6 months, using at least 3 samples that span entire measurement range • Not required separately if at least 3 calibrators that span range used
  • 29. 29 Analytical Measurement Range • For PCO2, PO2, difficult to get patient samples that span measurement range, and these are not stable on storage • Various vendors (including CAP) produce linearity materials that can be used to verify AMR; in our lab, we use CAP LN-13 survey for this purpose
  • 30. 30 Analytical Measurement Range • For most recent survey, ranges covered were: – pH: 6.8-8.8, allowed difference ± 0.3% – PCO2: 12-91.5, allowed difference ± 5% – PO2: 37-445, allowed difference ± 8% • Remember, values outside AMR must be reported > or < limits
  • 31. 31 Method Comparison • CHM.13800 requires that, if more than one method used for measuring same parameter, labs check against each other at least 2x yearly for patient results • Requires development of criteria for acceptable agreement
  • 32. 32 Method Comparison • Note indicates patient samples are preferable, but may not be possible for unstable analytes if instruments located at some distance (often an issue with blood gas instruments/samples) • If controls or reference material used, lab must verify they perform similarly to patient samples
  • 33. 33 Method Comparison • Does not require that single set of samples be used to compare all instruments; may compare instrument 1 and 2 with one set, instrument 1 and 3 with another set • Criteria should be based on patient care needs and/or criteria needed to pass proficiency testing
  • 34. 34 Method Comparison • Be careful to not use “blanket” criteria for whole laboratory; for example, in recent inspection I performed, acceptable agreement for all tests was ± 10% • For pH, at target value of 7.40, this would mean “acceptable” was between 6.66 and 8.14!
  • 35. 35 Method Comparison • For CAP proficiency testing (based on CLIA-88 regulations), acceptable performance is within: – pH: ± 0.04 – PCO2: ± 5 mm Hg or 8%, whichever is lower – PO2: ± 3 s.d.
  • 36. 36 Method Comparison • Question and note do not specify actual method to be used or number of samples; in our lab, we perform this analysis with 5 samples • Best to use a range of values (both normal and abnormal high and low) to detect any biases
  • 37. 37 Method Comparison • We use form, fill in test, ID of samples used, results from different instruments, and director review of acceptability • We have a single sheet for each section on allowable differences between methods developed by director, based on typical variation in patients (see Tietz or Kaplan & Pesce for tables)
  • 38. 38 Reference Intervals • CHM.15000 requires all results to be reported with reference intervals • With point of care instruments, printout often does not have this information on it, and is sometimes the only “report” available to clinicians • Lab must assure that some form with reference intervals is attached to report
  • 39. 39 Critical Values • CHM.15100 requires labs to establish criteria that require immediate notification • It is not required that all areas have such critical values, but it is common for blood gases to have critical values • May want to have separate critical values by age (for example, high PO2 dangerous in infants, not so in children, adults)
  • 40. 40 Critical Values • CHM.15200 requires documentation of prompt notification of physician • If blood gas lab is under separate leadership (e.g., respiratory medicine), important that similar approach used to what is done in central lab • Often, critical values are hospital, not laboratory, policies
  • 41. 41 Carryover • CHM.24400 requires labs to evaluate automatic pipetting systems for carryover • Since blood gas instruments often use pipetting systems, needs to be done for these instruments as well • Protocol involves running high sample several times, then repeat low sample
  • 42. 42 Carryover • Degree of difference for low sample (from when run previously) should be within lab’s criteria for acceptable difference between instruments • Seldom an issue in blood gas lab because of the limited range of results obtained, but needs to be done once
  • 43. 43 Quality Management • A number of questions on QM are found in lab general section • Remember that QM provisions apply to blood gas laboratory, and some QA monitor must be done if blood gas section is separate from main laboratory
  • 44. 44 Summary • If blood gas lab is part of main laboratory, there are few specific requirements • If separate section with different director (common situation), blood gas lab must comply with full Chemistry, Lab General, and Team Leader Checklist requirements
  • 45. 45 Summary • Important to note differences in how some questions apply to blood gas labs because of differences in type of samples and their stability • This session covered some of the most common areas where these differences are important