Pathology: Selecting And Interpreting Lab Results

Front 1

What question should you always ask yourself when ordering a lab test?

Back 1

"Will the result of this lab test have an effect on how I proceed?"

Front 2

What defines a reference interval?

Back 2

A reference interval is the central 95% of test values for a population of healthy individuals with characteristics comparable to those of the patient.

Front 3

Give three examples of factors that might cause there to be more than one reference interval for a test.

Back 3

Age, race, gender

Front 4

Name four lab tests that only have one reference range.

Name four that have different reference ranges for certain groups within the population.

Back 4

Sodium, potassium, calcium, and albumin only have one reference range for the population.

Urea, creatinine, enzymes, and hormones have different reference ranges for different groups within the population.

Front 5

What physiological function do serum creatinine levels reflect?

Back 5

Serum creatinine reflects the glomerular filtration rate. High values reflect slow glomerular filtration.

Front 6

What conditions lead to lower basal serum creatinine levels?

Back 6

Conditions involving loss of muscle mass, such as muscle wasting disorders or amputation.

Front 7

What disorders does serum creatine kinase level help to diagnose?

Does elevation or depression signal the presence of one of these problems?

Back 7

Myocardial infarction, muscle disorders

Elevation

Front 8

Disorders of what two organs are indicated by abnormal alkaline phosphatase levels?

What factor in a population requires stratification of reference intervals?

Back 8

Liver and bone

Age

Front 9

What are the chances of a single lab result occurring outside the reference interval for a healthy individual?

Back 9

5%

Front 10

Name four possible causes of false positives in test results.

Back 10

1. A reference interval from the wrong population is being used.

2. There is interference with the analysis from drugs, toxins, metabolites, hemolysis, icterus, lipeia, etc.

3. Physiological factors such as diurnal variation, eating, exercising, etc. are interfering.

4. There is just a statistical abnormality. (Actually falling outside the reference interval while healthy.)

Front 11

What is the pre-test probability of a disease?

Back 11

The pre-test probability of the disease is the likelihood that the patient will have the disease, before any test has been done. (ie: prevalence)

Front 12

Define the sensitivity of a test.

Back 12

The sensitivity is the probability of a positive test result if the disease is present.

Front 13

Define the specificity of a test.

Back 13

The specificity is the probability of a negative test result if the disease is absent.

Front 14

What is the predictive value positive?

Back 14

The PVP is the number of true positives divided by the sum of the true and false positives. It is the probability that someone with a positive test result has the disease.

Front 15

What is the predictive value negative?

Back 15

The PVN is the number of true negatives divided by the sum of the true and false negatives. It is the probability that someone with a negative test result does not have the disease.

Front 16

What factors must be taken into account when evaluating the value of a test?

Back 16

Sensitivity and specificity are key factors, but one also must factor in the prevalence, which affects the PVP and the PVN. If a disease has low prevalence, then the specificity must be high to avoid too many false positives.

Front 17

Generally, what does Bayesian analysis relate to?

Back 17

Bayesian analysis relates to examination of the predictive value of a test.

Front 18

Are positive test results for a low prevalence disease more likely to be true or false positives?

Back 18

They are more likely to be false positives.

Front 19

What are the three functional categories that lab tests can be divided into?

Back 19

1. Discovery: Does the patient have the disease?
2. Confirmation: Is it certain that the patient has the disease?
3. Exclusion: Is it certain that the patient does not have the disease?

Front 20

What quality is essential in a discovery test?

Give four examples of discovery tests.

Back 20

High sensitivity is essential for a discovery test.

1. Serum TSH- hypo-hyperthyroidism
2. Serum calcium- primary hyperparathyroidism
3. Urine glucose- diabetes mellitus
4. Hematologic profile- anemia

Front 21

What quality is essential in a confirmatory test?

Give three examples of confirmatory tests.

Back 21

Specificity must be 100% for a confirmatory test.

1. Low serum T4 in the presence of high TSH- primary hypothyroidism
2. High serum PTH- primary hyperthyroidism in patients with hypercalcemia
3. Positive cardiac troponin- acute MI in a patient with chest pain

Front 22

What quality is essential in an exclusion test?

Give three examples of exclusion tests.

Back 22

Sensitivity must be 100% for an exclusion test.

1. Normal serum TSH- primary hypotheyroidism
2. Absense of serum cardiac troponin after chest pain- acute MI
3. Normal glucose tolerance test- diabetes mellitus in a patient with glucose in their urine

Front 23

Explain the concept of a decision limit.

Back 23

A decision limit is the value that is determined as the cutoff between normal and abnormal values. By adjusting the decision limit, specificity and sensitivity are inversely affected.

Front 24

Define accuracy in terms of lab tests.

Back 24

The accuracy of a lab test describes the closeness of the test result to the true value.

Front 25

Define precision in terms of lab tests.

Back 25

The precision of a lab test describes the reproducibility of test results on the same specimen.

Front 26

What might make a lab test more or less accurate?

Back 26

The method of assessment makes tests more or less accurate. Direct measurement methods, such as using ion selective electrodes, radioimmunoassays, or chemiluminescent assays, are more accurate. Indirect measurement methods, such as measuring the reaction of creatinine with alkaline picrate are less accurate because other compounds in the blood might react with alkaline picrate and the proportion of these compounds present in the blood may vary with different conditions.

Front 27

What is the Coefficient of Variation (CV)?

Back 27

The CV is defined as standard deviation of test results divided by the mean of the test results, times 100. It is a measurement of the degree of imprecision of a lab test.

Front 28

What is the rule of thumb regarding changes in lab results over time and CV?

Back 28

Lab results must differ by at least 3xCV to be considered a true change, in order to rule out chance variation in test results.

Front 29

What is a major drawback to ordering a barrage of lab tests?

Back 29

Besides the high cost, the more tests you order, the more statistically likely you are to have a red herring test result that is abnormal, despite an absence of relevant pathology.