Laboratory medicine basic qc concepts
Sponsored Links
This presentation is the property of its rightful owner.
1 / 35

Laboratory Medicine: Basic QC Concepts PowerPoint PPT Presentation


  • 68 Views
  • Uploaded on
  • Presentation posted in: General

Laboratory Medicine: Basic QC Concepts. M. Desmond Burke, MD . Laboratory Error, “Normal Ranges,” & Predictive Values. M. Desmond Burke, M.D. Weill Cornell Medical Center New York, New York . Laboratory Diagnosis. laboratory error - preanalytical - analytical: accuracy & precision

Download Presentation

Laboratory Medicine: Basic QC Concepts

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Laboratory Medicine: Basic QC Concepts

M. Desmond Burke, MD


Laboratory Error, “Normal Ranges,” & Predictive Values

M. Desmond Burke, M.D.

Weill Cornell Medical Center

New York, New York


Laboratory Diagnosis

  • laboratory error- preanalytical- analytical: accuracy & precision

  • “normal” or “reference” values

  • sensitivity, specificity & prevalence

  • predictive value

  • pretest & posttest probabilities

  • thresholds & test strategy


CLINICALCLUES

TEST STRATEGIES

HYPOTHESIS

ACTIVATION

HYPOTHESIS

REVISION

ROUTINE TESTS

PATIENT MANAGEMENT


CLINICAL VALUE

DIAGNOSTIC VALUE

TECHNICAL RELIABILITY


Action

Test selection

Interpretation

Question

Reporting

Ordering

Analysis

Identification

Preparation

Collection

Transportation


Laboratory Error: Preanalytical

  • patient preparation- diet, activity

  • specimen collection- wrong name, wrong tube, wrong time wrong technique

  • specimen transport & storage- delays, wrong temperature


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


Precise& Inaccurate

Inaccurate & Imprecise

Precise & Accurate

o

o

o

o

o

o

o

oo

o

o

o

o

o

o

o

Accurate & Imprecise


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


“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


95%

68%

#

Mg/dL


Relationship of “Expected Abnormal Results” to Number of Measured Constituents

Probability of abnormal result: 1 – 0.95n : n equals test number


CLINICAL VALUE

DIAGNOSTIC VALUE

TECHNICAL RELIABILITY


POST-TEST PROBABILITIES

POST-TEST PROBABILITY,

GIVEN A POSITIVE TEST RESULT = PV+

POST-TEST PROBABILITY,

GIVEN A NEGATIVE TEST RESULT = 100 – PV-


TEST

TREAT

DO NOT TEST

DO NOT TEST

DO NOT TREAT

1.0

0.0

RULE-IN

THRESHOLD

RULE-OUT THRESHOLD

PROBABILITY OF DISEASE


POSTTEST NEG (P)

PRETEST (P)

POSTTEST POS (P)

10

50

90

TEST

1.0

0.0

RULE-IN

THRESHOLD

RULE-OUT THRESHOLD

PROBABILITY OF DISEASE


Sensitivity & Specificity

  • SENSITIVITY- the percentage of diseased individuals with abnormal test results

  • SPECIFICITY- the percentage of healthy individuals with normal results


False Negatives & False Positives

  • FALSE NEGATIVE RATE- 100 minus SENSITIVITY

  • FALSE POSITIVE ATE- 100 minus SPECIFICITY


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


TEST

RESULT

DISEASE

PRESENT

DISEASE

ABSENT

POSITIVE

TP

FP

POSTTEST PROBABILITY

GIVEN A POSITIVE RESULT:

TP/TP+FP

NEGATIVE

FN

TN

POSTTEST PROBABILITY

GIVEN A NEGATIVE RESULT:

FN/TN+FN

TOTALS

TP + FN

TN + FP

SENSITIVITY

TP/TP+FN

SPECIFICITY

TN/FP+TN


POSTTEST PROBABILITY OF DISEASE WHEN PRETEST PROBABILITY IS 50%

TEST

RESULT

DISEASE

PRESENT

DISEASE

ABSENT

POSITIVE

90

10

POSTTEST PROBABILITY

GIVEN A POSITIVE RESULT:

90/100

(90%)

NEGATIVE

10

90

POSTTEST PROBABILITY

GIVEN A NEGATIVE RESULT:

10/100 (10%)

TOTALS

100

100

SENSITIVITY

90%

SPECIFICITY

90%


POSTTEST NEG (P)

PRETEST (P)

POSTTEST POS (P)

10

50

90

TEST

1.0

0.0

RULE-IN

THRESHOLD

RULE-OUT THRESHOLD

PROBABILITY OF DISEASE


POSTTEST PROBABILITY OF DISEASE WHEN PRETEST PROBABILITY IS 90%

TEST

RESULT

DISEASE

PRESENT

DISEASE

ABSENT

POSITIVE

810

10

POSTTEST PROBABILITY

GIVEN A POSITIVE RESULT: 810/820 (99%)

NEGATIVE

90

90

POSTTEST PROBABILITY

GIVEN A NEGATIVE RESULT:

90/180 (50%)

TOTALS

900

100

SENSITIVITY

90%

SPECIFICITY

90%


POSTTEST PROBABILITY OF DISEASE WHEN PRETEST PROBABILITY IS 10%

TEST

RESULT

DISEASE

PRESENT

DISEASE

ABSENT

POSITIVE

90

90

POSTTEST PROBABILITY

GIVEN A POSITIVE RESULT:

90/180

(50%)

NEGATIVE

10

810

POSTTEST PROBABILITY

GIVEN A NEGATIVE RESULT:

10/820 (1%)

TOTALS

100

900

SENSITIVITY

90%

SPECIFICITY

90%


Effects of Prevalence

  • DECREASING PREVALENCE- decreases (PV+) & increases (PV-)

  • INCREASING PREVALENCE- increases (PV+) & decreases (PV-)


F

D

D

X

Xc


5

10

12

15

D

D

X

Xc


RECEIVER-OPERATOR CHARACTERISTIC (ROC) CURVES

100

BEST

BETTER

% TP

50

USELESS

100

0

50

% FP


  • 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


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

KEY IS: SPECIFICITY

KEY IS: SENSITIVITY

POSTTEST PROBABILITY (+)

SHOULD BE HIGHER THAN “RULE IN” THRESHOLD

POSTEST PROBABILITY (-)

SHOULD BE LOWER THAN THE

“RULE OUT” THRESHOLD


POSTTEST PROBABILITY OF CHD WHEN PRETEST PROBABILITY IS 10%

TEST

RESULT

DISEASE

PRESENT

DISEASE

ABSENT

POSITIVE

86

207

POSTTEST PROBABILITY

GIVEN A POSITIVE RESULT:

86/293

(29%)

NEGATIVE

14

693

POSTTEST PROBABILITY

GIVEN A NEGATIVE RESULT:

14/707 (2%)

TOTALS

100

900

SENSITIVITY

86%

SPECIFICITY

77%


POSTTEST PROBABILITY OF CHD WHEN PRETEST PROBABILITY IS 90%

TEST

RESULT

DISEASE

PRESENT

DISEASE

ABSENT

POSITIVE

774

23

POSTTEST PROBABILITY

GIVEN A POSITIVE RESULT:

774/797

(97%)

NEGATIVE

126

77

POSTTEST PROBABILITY

GIVEN A NEGATIVE RESULT:

126/203 (62%)

TOTALS

900

100

SENSITIVITY

86%

SPECIFICITY

77%


POSTTEST NEG (P)

PRETEST (P)

POSTTEST POS (P)

90

62

97

TEST

1.0

0.0

RULE-IN

THRESHOLD

RULE-OUT THRESHOLD

PROBABILITY OF DISEASE


POSTTEST

NEG (P)

PRETEST (P)

POSTTEST POS (P)

1

10

50

TEST

1.0

0.0

RULE-IN

THRESHOLD

RULE-OUT THRESHOLD

PROBABILITY OF DISEASE


  • Login