qc delta pv ppt student 09

8/14/2019 QC Delta PV Ppt Student 09

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CLS 500 Application and Interpretation of Clinical Laboratory Data

QC, Delta Check and Predictive Value Lecture

Quality Control, Delta Checks

and Predictive Value

CLS 500 Application and Interpretation of ClinicalLaboratory Data

Ricki Otten, MT(ASCP)SC

[email protected]

2

1. Discuss the use of quality control in

ensuring the validity of laboratory testresults

2. Define the acceptable range as it

relates to quality control and how this

range is determined.

Objectives

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3. Define the terms as they relate to quality

control:

a. Accuracy

b. Precision

c. Reliability

4. Explain the use of delta checks to

assure accurate patient test results

Objectives

4

Objectives

5. Define the following terms:

a. Test sensitivity

b. Test specificity

c. Predictive value

6. Discuss how sensitivity, specificity and

predictive value are used to assess the

diagnostic usefulness of a test

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Who needs it?

What is it?

Why is it done?

How is it used?

Quality Control (QC)

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The most basic and fundamental

expectation of a laboratory is


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QC is a statistical tool that is used to

assure the reliability of test results

Statistical quality control practices are

critical for assuring test results are correct

Quality Control (QC)

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QC: What is it?

The value of the quality control specimen isknown

and is represented by an acceptable range of

values that has been statistically calculated

Acceptable range = Mean 2 SD

For each control level for each assay:

1. Control is analyzed repeatedly on separate days

2. Values are collected until 30+ values are obtained

3. Statistics are calculated: mean, SD, %CV

4. Acceptable range is determined

9Acceptable range = Mean 2SD = 6.8 0.6 = 6.2 7.4 g/dl 10

QC statistically validates the accuracy

(correctness) of

Methodology

Testing procedure

Reagents Analytical Run

Entire test system

Instruments

QC: What Does it Do?

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QC cannot determine if the sample

submitted is from the correct patient!

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The laboratory wants to report the mostaccurate and precise values

QC samples are analyzed along with thepatient samples

Evaluation of QC results determines thevalidity of the analytical run

If analytical run is valid, then we are confidentthe patient results are also valid

QC: How is it Used?


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If QC results are within acceptable limits

then the analytical run is validated as accurate.

This means the patient results are valid and can

be reported

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If QC results are not within acceptable limitsthen the analytical run is not valid.

This indicates an analytical error has occurred that

jeopardizes the patient results.

This means the patient results may not be valid

and cannot be reported.

After troubleshooting the error, all samples must

be reanalyzed and QC must be statistically

re-evaluated before patient test results are reported16

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What does accuracy and precision mean?

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Accuracy refers to the closeness of a

measurement to the true value

Precision refers to the reproducibility of a

measurement

Reliability refers to the accuracy and

precision of a measurement

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Quality Control Exercise

Evaluate the following analytical run in

relation to test validity.

Should the patient test results be released

to the medical record?

Why or why not?

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69

150

43

132

97

1. Level I control

2. Level II control

3. John Smother

4. Gomer Pyle

5. Jayne Sealess

Result (mg/dl)Analytical Run: plasma glucose

162 198 mg/dl180 +/-9 mg/dlLevel II

62 78 mg/dl70 +/-4 mg/dlLevel I

Acceptable

RangeMean +/- 2SD

Quality Control Acceptable Limits

Critical low: < 45 mg/dl

Critical high: > 450 mg/dl

Fasting plasma glucose: 70-99 mg/dl

Expected Reference Range and Critical

Limits

Is level I control within acceptable limits?Is level II control within acceptable limits? 22

69 OK

150 Not OK

43

132

97

1. Level I control

2. Level II control

3. John Smother

4. Gomer Pyle

5. Jayne Sealess



62 78 mg/dl70 +/- 4 mg/dlLevel I

Acceptable

RangeMean +/- 2SD





Expected Reference Range and Critical

Limits

Can patient test results be reported? Why/why not?

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69 OK

150 Not OK

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132

97

1. Level I control

2. Level II control

3. John Smother

4. Gomer Pyle

5. Jayne Sealess



62 78 mg/dl70 +/-4 mg/dlLevel I

Acceptable

RangeMean +/- 2SD





Expected Reference Range and CriticalLimits

Can patient test results be reported? Why/why not?

No; the validity of the analytical run is questioned because

control level II is outside of acceptable limits. All testing must

be repeated and when all controls are within acceptable limits,

the patient test results may be reported.24

Delta Check

Another tool used by the laboratory to

assure accurate patient test results for

specific analytes

The most recent test result for a particular

patient is compared to the most previous

test result for that patient


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Delta Check

If the test result has changed significantlyfrom the last time the test was done, a

delta check failure occurs

The delta check failure means the changein test results is not physiologicallypossible

The allowable difference is pre-determined by various investigators andclinicians 26

Delta Check

The technologist is now responsible to

determine the cause of the delta check

failure before releasing test results

Test sample may have been compromised

Test interference: hemolyzed, lipemic, icteric

Sample not handled properly

Drawn at incorrect time: trough, peak, tolerance

Drawn in incorrect sample tube: B-R-G-P-G

Treatment?

Is this the correct patient?

Redraw the specimen?

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Jan.1 Jan.8 Jan.12 Jan. 13

BUN 88 95 105 22*

(mg/dl)

*Delta check failure

What is a likely cause for this failure?

Should test results be reported?28

Jan.1 Jan.2 Jan.3 Jan.4

K+ 3.9 3.6 4.1 6.9*

(mmol/L)

*Delta check failure

What is a likely cause for this failure?

Should test results be reported?

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Diagnostic Efficacy

Diagnostic sensitivity

Diagnostic specificity

Positive predictive value Negative predictive value

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Ideal

All persons with disease will test positive

(referred to as diagnostic sensitivity)

All persons without disease will test negative

(referred to as diagnostic specificity)

Test results would be: 100% sensitive

100% specific


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Reality

All methods have an inherent amount of

error that will affect test results

No method is able to detect all persons

with disease accurately -and-

No method is able to detect all persons

without disease accurately

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Possible Test Outcomes

- (TN)- (FN)

Negative testresult

+ (FP)+ (TP)

Positive test

result

Patients withoutdisease

Patients withdisease

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- (TN)- (FN)

Negative testresult

+ (FP)+ (TP)

Positive test

result

Patients withoutdisease

Patients withdisease

Test sensitivity: TP/(TP + FN) x 100

Test specificity: TN/(TN + FP) x 100

Pos predictive value: TP/(TP + FP) x 100

Neg predictive value: TN/(TN + FN) x 100 34

Hypothetical Example

A total of 3000 patients were assessed for

acute myocardial infarction (AMI) with

Troponin I (cTnI) measurements:

1000 patients tested positive for cTnI and

of those, 980 had diagnostic AMI;

2000 patients tested negative for cTnI and

of those, 30 had diagnostic AMI

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Determine the sensitivity, specificity,

positive predictive value (+PV) and

negative predictive value (-PV) of the

test used for assessment of AMI

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Construct 2x2 Table

Neg test

result

Pos test

result

Patientswithout AMI

Patientswith AMI


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Calculate Test Sensitivity

Test sensitivity: TP/(TP + FN) x 100

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Calculate Test Specificity

Test specificity: TN/(TN + FP) x 100

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Calculate Pos Predictive Value

Pos predictive value: TP/(TP + FP) x 100

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Calculate Neg Predictive Value

Neg predictive value: TN/(TN + FN) x 100

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PSA values >4.0 ng/ml

Sensitivity = 79% Specificity = 46%

+PV = 33% -PV = 87%

265 (TN)41 (FN)

Neg test result

(4.0 ng/ml)

Patients withoutprostate cancer

Patients withprostate cancer

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High Cut-off Value

If a high cutoff value is used,

then the specificity of the test can reach100%

However, the sensitivity will decrease


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Low Cut-off Value

If a low cutoff value is used,

then the sensitivity of the test can reach

100%

However, the specificity will decrease

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Disease PrevalenceUsing PSA cut off value of 4.0 ng/ml:

Sensitivity = 79%; Specificity = 46%; +PV = 33%; -PV = 87%)

59.4%0.5 (50% of pop)

26.8%0.2 (20% of pop)

14.0%0.1 (10% of pop)

1.5%0.01 (1% of pop)

0.1%0.001 (0.1% of population)

Pos Predictive ValuePrevalence of disease

qc delta pv ppt student 09

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