Lecture 4 - Lab Test Stewardship

why might a pharmacist need to order a laboratory test

• ensure drug and dose are appropriate

• monitor patient response to therapy

• monitor for adverse effects and ensure patient safety

• screen patients with preliminary indicators for untreated disease

what does a pharmacist need to do to order a laboratory test

register with the laboratory (Alberta Precision Laboratories) - receive a code

be familiar with the standards and responsibilities

plan for receiving/interpreting results (need 24/7 contact info in case of critical results)

things to consider when ordering a lab test

is it appropriate

is the cost of test worth the information it generates

logistical issues for the patient (e.g. fasting, collect and deliver samples)

what should a pharmacist always do before initiating lab requests and POCT

check patient’s netcare

what is informed consent for laboratory tests

patients must be provided with the name of the test, the reason for testing, the risks and benefits of the test, and what the plan for follow up is

what are general principles when interpreting lab results

accuracy and precision

test performance

reference range

critical value

units

what is accuracy

how close is a measurement to its true value

measurements are never absolute

influence of both random and systematic error

what is precision

represents degree of arrangement between multiple measurements of the same sample when repeatedly measured under same conditions

what is sensitivity of a lab test

related to error

The ability of a test to correctly identify people who have a disease/condition

what does it mean if a lab test has high sensitivity

high possibility of a true positive

low possibility of a false negative

someone with a negative result does not have disease/condition

what does it mean if a lab test has low sensitivity

a negative result could lead to missing a case of disease

what is specificity of a lab test

ability of a test to correctly identify people who truly do not have a given condition

what does it mean if a test has high specificity

low possibility of false positive

high possibility of true negative results

high confidence someone with positive result has the condition/disease

what does it mean if a test has low specificity

a positive result could lead to unnecessary additional tests/over treatment

what if a test is too sensitive

increase the chance of getting false positive results

what if a test is too specific

increase the chance of false negative results

what are predictive values

provides more clinically meaningful information for individual patient care

What is a positive predictive value

percent of positive results that are truly positive

if the test is positive, what is the probability the patient actually has the disease

what is negative predictive value

percent of negative results that are truly negative

if the test is negative what is the probability the patient does not actually have the disease

what is the formula for sensitivity using a 2×2 table

true positive / patients with disease

what is the formula for specificity using a 2×2 table

true negative / patients without disease

what is the formula for PPV (positive predictive value) using a 2×2 table

true positive / all positive test results

what is the formula for NPV (negative predictive value) using a 2×2 table

true negative / all negative test results

what is a reference range

range of values obtained from testing a large sample of health individuals

range will contain 95% of the values

reference range may vary from lab to lab

what are critical values

test result is outside of the reference range and requires immediate attention

first: possibility of error?

how far is critical value beyond reference range, are patient symptoms consistent with the lab value

what units are used for lab tests

Canada: SI (international units)

mmol/L, mEq/L

what are the steps to follow when interpreting lab tests

1. gather patient information

2. look at netcare for relevant lab results

3. if not available → request/order lab test

4. compare the lab result with reference range

5. assess lab result with patient symptoms, medical conditions, other influential factors

6. determine if action is required