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Cellular Week 3 > Ordering Lab Test I > Flashcards

Ordering Lab Test I Flashcards

1
Q

PSA (prostate specific antigen)

A

Protease produced by glands of the prostate

Elevated in prostate cancer (correlated with tumor burden in metastatic prostate cancer)

Can be elevated in patients without prostate cancer (benign prostatic hyperplasia)

Assayed with capture immunoassay, using a chemiluminescent reaction to quantitate the PSA serum level

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2
Q

A quantity that describes accuracy is….?

A

Bias

The difference between the “true” value and the mean of the measured values is the bias

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3
Q

What is the quantity that describes precision?

A

% cv (coefficient of variation)

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4
Q

What is the %cv?

A

Standard deviation divided by the mean x 100

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5
Q

Total error?

A

The percent bias times percent precision

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6
Q

Total allowable error?

A

Takes into account clinical decision making - e.g. Break points for clinical action

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7
Q

Functional sensitivity

A

Level at which the %cv does not exceed 20% (high cv is bad - talks about precision)

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8
Q

The lower end of the AMR (analytical measurement range) is defined by….

A

The functional sensitivity, provided the linearity at that level is acceptable

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9
Q

The upper end of the AMR (analytical measurement range) is defined by…

A

The limit of linearity

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10
Q

When should the AMR extend down to a really low value….

A

Monitoring FSH and LH in children with precocious puberty (being treated with GNRH)

Hospital based labs use 2nd generation and national reference labs use 3rd generation - both are more sensitive

Or!! HCG monitoring for females of reproductive age taking isotretinoin for severe acne or lenalidomide for multiple myeloma

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11
Q

CRR (clinical reporting range)

A

Typically the same as the AMR at the lower end, but can be much higher at the upper end

This is fixed with the use of a validated dilution protocol for the pt samples that exceed the AMR

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12
Q

Exs. Of when the CRR should be higher than the upper range of the AMR?

A

Tumor markers

Creatine Kinase (CK) - protein released from muscle fibers that are necrotic
- pts with duchenne muscular dystrophy
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13
Q

What does analytical specificity ask?

A

Does the assay actually measure what it purports to measure and nothing else?

  • for immunoasssays, you have to be concerned about cross-reactive substances and interferences
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14
Q

Reference Range - population based

A

Mean +/- two standard deviations of normal subject data

Central 95% of normal subject data

Normally the population is somewhat skewed

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15
Q

Reference range: physiologic

A

Rarely available!

Ex. PTH - vitamin D level in serum is construed as the lvl at which PTH plateaus

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16
Q

Reference range - based on health

A

Based not upon the “normal” population but rather on what is considered optimal for health

Ex. American Heart association considers plasma cholesterol should not exceed 200 mg/dl

Arbitrary cut off

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17
Q

Few ideal lab tests exist where the normal and diseased states do not overlap with respect to analyze values

A

…

18
Q

“False results” do not refer to flawed assays but rather mismatches between disease status and analyte values

A

…

19
Q

Sensitivity formula?

A

TP/TP+FN

20
Q

Specificity formula?

A

TN/TN+FP x 100

21
Q

Sensitivity refers to?

A

How many people with the disease have abnormal results

22
Q

Specificity refers to?

A

How good is this test? Does this test really tell us that they have it?

Defined by people who don’t have the disease and how many of them are negative for that marker

23
Q

Screen with a test for which the cut off has what first? Then what second?

A

Has been designed for high sensitivity first (get all positives) and then confirm the test for which the cut-off has been designed for high specificity (find all the true negatives)

Ex. Borrelia burgdorferi antibody screen with ELISA then with Western Blot/PCR

24
Q

Receiver operating characteristic (ROC) curve

A

Test performance is correlated with area under the curve

Cut off value depends upon use:

1) screening (max sensitivity at the expense of specificity)
2) risks associated with false diagnosis

What to be near the top left

25
Q

Positive predictive value formula?

A

TP/TP +FP

26
Q

Negative predictive value formula?

A

TN/TN+FN

27
Q

What is on the predictive value table?

A

Test result (pos/neg)

Disease (present/absent)

28
Q

The very same test can perform better or worse depending upon the prevalence of the disease that the test is used to diagnose…. for ex?

A

Hospitalized pts have typically higher prevalence of disease than out patients, thus have PPV higher and NPV is higher for out patients

29
Q

Disease Probability: Bayes’ Theorem

A

“Pre-test probability” may be thought of as the prevalence of disease; ideally it is refined with additional info such as hist/PE

30
Q

Likelihood ratio - positive test

A

For a pos test, it is the probability of a person with the disease testing positive, divided by the prob of a person without diesese testing positive (i.e. The ratio of true positive rate to false positive rate)
Sensitivity/(1-specificity)

31
Q

Likelihood ratio - neg test

A

Prob of a person with the disease testing negative divided by the prob that a person without the disease testing negative - i.e. , the ratio of the false negative ration to the true negative ratio

1-sensitivity/ specificity

32
Q

Post test probability

A

Can be derived from a nomogram

33
Q

Liklihood ratios for a positive result are better for…?

A

Diagnosing disease the higher they are (over 10)

34
Q

Likelihood ratios for a negative result are better at….

A

Ruling out diseases the lower they are

35
Q

Exs where Bayes’ theorem can be applied without the help of a computer are oversimplified: they typically treat tests as qualitative (pos or neg) and they are focused on one test

Computer prediction models that treat test results as continuous quantitative variables and that take into account multiple test results (multi-variant analysis) are the future tools you will use; there is some clinical use already with these tools

A

…

36
Q

Within lab test performance characteristics dictate the analytical range for an assay

A

…

37
Q

Between lab differences exist for accuracy, precision, and reference range for tests

A

…

38
Q

ROC curves can be used to…

A

Compare test and also to find optimal decision cut offs

39
Q

Disease prevalence figures into test utility

A

…

40
Q

Pre and post test probabilities shoudl considered when entertaining a diagnosis or a diagnostic plan

A

….

Cellular Week 3 (21 decks)

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