Path: Use of the Lab I Flashcards
What are the roles of the clinical laboratory?
screening, diagnosis, follow up, prognosis
What can increase the predictive value of a screening test?
increase prevalence by selecting target pop likely to have disorder on which test is to be performed
What can increase the sensitivity and specificity of a screening test?
using combinations of tests (4 markers instead of 3 on maternal risk for fetal anomalies)
positive screening initiates confirmatory testing
What are examples of screening tests?
phenylalanine levels in newborns for PKU
TSH levels in newborn screen for hypothyroidism
glucose and lipid tests for annual exam for DM and dyslipidemias
PSA for prostate cancer
What does the use of laboratory tests in diagnosis require?
ID of reference population, target test value or decision level
What must a monitoring test be for effective follow up?
disease sensitive
What are things to consider when choosing a laboratory test for prognosis?
concordance w clinical findings
indication for and frequency of monitoring
sensitivity and specificity
What are the three phases of the laboratory testing cycle?
pre-analytical
analytical
post-analytical
What are the components to the pre-analytical phase?
decision to perform the test test order placed patient identifying information entered patient prep, specimen collection accounts for majority of lab errors
What are the components to the analytical phase?
specimen processed
analyte measured
What are the components to the post-analytical phase?
report generated
result/interpretation conveyed to clinician
data interpreted
clinical response to result
What types of patient prep might be necessary for specimen collection?
fasting
posture
dietary factors, alcohol intake, and physical exercise
stress can affect results
What are pre-analytical factors influencing laboratory results?
time of collection (patient prep and sample collection)
test selection
specimen type (additives)
clinical info and list of therapeutics (other drugs on board, etc)
What are important characteristics of the analytical phase?
sensitivity (detection limit) - lowest concentration the assay can detect or reliably distinguish from absence
specificity (interference) - determines degree of interference from other similar anlaytes, metabolites, or other endogenous material such as drugs or inhibitors
results must be reproducible (precise)
measuring range of assay (linearity) must span the clinically useful and thus relevant reportable range
accuracy
What are positive and negative predictive values?
positive = fraction of subjects w a positive test who have the dz negative = fraction of subjects w a negative test who do not have the dz
What types of analytical errors are there?
random: caused by timing, temp, pipetting variations
systematic: constant or proportional error, can be caused by shifting of the calibration curve
What can interfering substances do to a chemical reaction?
inhibit it = negative interference
enhance it or case increased detection signal without participating = positive interference
What are specific interferences in laboratory testing?
drug metabolites
spectral: hemolysis, lipemia, bilirubin
antibodies: human anti-mouse Abs (variable prevalence, strong affinity), heterophilic Abs (weak binding, immunological response to inf, rheumatoid factor)
What are ways to prevent interference due to heterophile antibodies?
test pt’s sample using different assay method
perform a dilution study for linearity
use blocking agent to inactivate heterophile Ab
What are post-analytical errors?
reference ranges
critical values: must be determined so they can be reported immediately
interpretive comment: ddx, suggestions for additional testing, recommendations for follow-up, opinion about possible interferences, limitations of test utility
