Laboratory Medicine: Basic QC Concepts
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Transcript of Laboratory Medicine: Basic QC Concepts
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Laboratory Medicine: Basic QC Concepts
M. Desmond Burke, MD
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Laboratory Error, “Normal Ranges,” & Predictive Values
M. Desmond Burke, M.D.Weill Cornell Medical Center
New York, New York
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Laboratory Diagnosis• laboratory error
- preanalytical- analytical: accuracy & precision
• “normal” or “reference” values• sensitivity, specificity & prevalence• predictive value• pretest & posttest probabilities• thresholds & test strategy
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CLINICAL CLUES
HYPOTHESIS ACTIVATION
ROUTINE TESTS
TEST STRATEGIES
HYPOTHESIS REVISION
PATIENT MANAGEMENT
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TECHNICAL RELIABILITY
DIAGNOSTIC VALUE
CLINICAL VALUE
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Action
Interpretation
Reporting
Preparation
Transportation
Question
Test selection
Ordering
Identification
Collection
Analysis
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Laboratory Error: Preanalytical• patient preparation
- diet, activity• specimen collection
- wrong name, wrong tube, wrong time wrong technique
• specimen transport & storage- delays, wrong temperature
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Accuracy & Precision
• Accuracy: “closeness to truth”- maintained routinely by calibrators - checked by inter-laboratory surveys
• Precision: “ reproducibility”- estimated by Standard Deviation (SD) or Coefficient of Variation (CV)- monitored by quality control sera
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oooo
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oPrecise& InaccuratePrecise & Accurate
Accurate & Imprecise
Inaccurate & Imprecise
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Importance of Quality Control to the Physician
• QUESTION:when is the difference between two successive test results within the limits of analytical imprecision?
• ANSWER:when the results differ by more than 3 x SD of the laboratory method
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“Normal” or “Reference”• “reference”is the appropriate word
- central 95 percent of the range of values in an apparently healthy population
• “normal” could mean: - free of neurosis, usual, ideal, free of
disease, or including the central 95 percent of a “normal” or gaussian distribution
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#
Mg/dL
95%
68%
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Relationship of “Expected Abnormal Results” to Number of Measured Constituents
Number of Measured Constituents
Expected % of one or more “abnormal” Results
1 52 104 196 2610 4015 5420 64
Probability of abnormal result: 1 – 0.95n : n equals test number
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TECHNICAL RELIABILITY
DIAGNOSTIC VALUE
CLINICAL VALUE
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POST-TEST PROBABILITIES
POST-TEST PROBABILITY,GIVEN A POSITIVE TEST RESULT = PV+
POST-TEST PROBABILITY,GIVEN A NEGATIVE TEST RESULT = 100 – PV-
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RULE-OUT THRESHOLD
RULE-IN THRESHOLD
0.0 1.0
PROBABILITY OF DISEASE
DO NOT TESTDO NOT TREAT
TEST TREATDO NOT TEST
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RULE-OUT THRESHOLD
RULE-IN THRESHOLD
0.0 1.0
PROBABILITY OF DISEASE
TEST
50
PRETEST (P)POSTTEST NEG (P)
10
POSTTEST POS (P)
90
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Sensitivity & Specificity
• SENSITIVITY- the percentage of diseased individuals
with abnormal test results
• SPECIFICITY- the percentage of healthy individuals
with normal results
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False Negatives & False Positives
• FALSE NEGATIVE RATE- 100 minus SENSITIVITY
• FALSE POSITIVE ATE- 100 minus SPECIFICITY
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Predictive Values
• POSITIVE PREDICTIVE VALUE (PV+)- the percentage of true positive test
results among all positive test results• NEGATIVE PREDICTIVE VALUE (PV-)
- the percentage of true negative test results among all negative test results
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TEST RESULT
DISEASE PRESENT
DISEASE ABSENT
POSITIVE TP FP
NEGATIVE FN TN
TOTALS TN + FP
SENSITIVITY
TP/TP+FN
SPECIFICITY TN/FP+TN
POSTTEST PROBABILITYGIVEN A POSITIVE RESULT : TP/TP+FP
POSTTEST PROBABILITYGIVEN A NEGATIVE RESULT: FN/TN+FN
TP + FN
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POSTTEST PROBABILITY OF DISEASE WHEN PRETEST PROBABILITY IS 50%
TEST RESULT
DISEASE PRESENT
DISEASE ABSENT
POSITIVE 90 10
NEGATIVE 10 90
TOTALS 100 100
SENSITIVITY 90%
SPECIFICITY 90%
POSTTEST PROBABILITYGIVEN A POSITIVE RESULT : 90/100 (90%)
POSTTEST PROBABILITYGIVEN A NEGATIVE RESULT: 10/100 (10%)
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RULE-OUT THRESHOLD
RULE-IN THRESHOLD
0.0 1.0
PROBABILITY OF DISEASE
TEST
50
PRETEST (P)POSTTEST NEG (P)
10
POSTTEST POS (P)
90
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POSTTEST PROBABILITY OF DISEASE WHEN PRETEST PROBABILITY IS 90%
TEST RESULT
DISEASE PRESENT
DISEASE ABSENT
POSITIVE 810 10
NEGATIVE 90 90
TOTALS 900 100
SENSITIVITY 90%
SPECIFICITY 90%
POSTTEST PROBABILITYGIVEN A POSITIVE RESULT : 810/820 (99%)
POSTTEST PROBABILITYGIVEN A NEGATIVE RESULT: 90/180 (50%)
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POSTTEST PROBABILITY OF DISEASE WHEN PRETEST PROBABILITY IS 10%
TEST RESULT
DISEASE PRESENT
DISEASE ABSENT
POSITIVE 90 90
NEGATIVE 10 810
TOTALS 100 900
SENSITIVITY 90%
SPECIFICITY 90%
POSTTEST PROBABILITYGIVEN A POSITIVE RESULT : 90/180 (50%)
POSTTEST PROBABILITYGIVEN A NEGATIVE RESULT: 10/820 (1%)
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Effects of Prevalence
• DECREASING PREVALENCE- decreases (PV+) & increases (PV-)
• INCREASING PREVALENCE- increases (PV+) & decreases (PV-)
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F
DD
XcX
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DD
XcX
5 10 12 15
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RECEIVER-OPERATOR CHARACTERISTIC (ROC) CURVES
% FP
0 10050
100
50% TPUSELESS
BETTER
BEST
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• DETERMINANTS OF SENSITIVITY- choice of cutoff or reference limit- severity of disease in patients chosen
to determine sensitivity- increased by multiple testing in parallel
• DETERMINANTS OF SPECIFICITY- choice of cutoff or reference limit- type of nondiseased persons chosen to
determine specificity - increased by multiple testing in series
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WHICH TEST TO ORDER?
WHAT IS THE PRETEST PROBABILITY OF THE PROBLEM?
WHAT ARE THE “RULE IN” & “RULE OUT” THRESHOLDS
TO RULE IN TO RULE OUT
POSTTEST PROBABILITY (+)SHOULD BE HIGHER THAN “RULE IN” THRESHOLD
POSTEST PROBABILITY (-) SHOULD BE LOWER THAN THE “RULE OUT” THRESHOLD
KEY IS: SPECIFICITY KEY IS: SENSITIVITY
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POSTTEST PROBABILITY OF CHD WHEN PRETEST PROBABILITY IS 10%
TEST RESULT
DISEASE PRESENT
DISEASE ABSENT
POSITIVE 86 207
NEGATIVE 14 693
TOTALS 100 900
SENSITIVITY 86%
SPECIFICITY 77%
POSTTEST PROBABILITYGIVEN A POSITIVE RESULT : 86/293 (29%)
POSTTEST PROBABILITYGIVEN A NEGATIVE RESULT: 14/707 (2%)
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POSTTEST PROBABILITY OF CHD WHEN PRETEST PROBABILITY IS 90%
TEST RESULT
DISEASE PRESENT
DISEASE ABSENT
POSITIVE 774 23
NEGATIVE 126 77
TOTALS 900 100
SENSITIVITY 86%
SPECIFICITY 77%
POSTTEST PROBABILITYGIVEN A POSITIVE RESULT : 774/797 (97%)
POSTTEST PROBABILITYGIVEN A NEGATIVE RESULT: 126/203 (62%)
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RULE-OUT THRESHOLD
RULE-IN THRESHOLD
0.0 1.0
PROBABILITY OF DISEASE
TEST
62
PRETEST (P)POSTTEST NEG (P) POSTTEST POS (P)
9790
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RULE-OUT THRESHOLD
RULE-IN THRESHOLD
0.0 1.0
PROBABILITY OF DISEASE
TEST
50
PRETEST (P)POSTTEST NEG (P)
1
POSTTEST POS (P)
10