Understanding Lab Tests Flashcards
Define what is meant by a reference range/interval.
Reference intervals generally represent 95% of the “normal” population
–Unless reference interval is established via decision limit/treatment guideline
State what percentage of normal individuals fall outside the reference range by definition.
5%, so 1/20
Explain what is meant by pre-analytic, analytic and post-analytic periods relative to laboratory tests. **Which of the above three is the most likely to lead to increased variation/error?
Pre-analytic: includes physician deciding to test, order entry, patient preparation, specimen collection, processing and transport
**MOST ERRORS occur here
Analytic: includes receiving specimen, analysis, and result verification
Post-analytic: includes physician acknowledging result, interpretation, and implementation of treatment course
Describe some of the problems leading to erroneous results that occur in the pre-analytic stage of the test.
Order entry errors: similar test names, duplicate orders, transcription errors
Patient preparation: fasting/not, exercise/position, timing of collection
Specimen collection: similar patient names, needle size (too small = hemolysis), tourniquet use/fist clenching (BAD for lab testing), urine preservatives
Processing and transport: delayed processing (-> decreased glucose, increased lactate, increased K), improper storage (freezing, overheating, room temp)
Explain the “Hook effect” in laboratory tests using an antigen-antibody system.
Hook Effect: significantly elevated concentrations of some analytes or measurands may produce falsely low concentrations
In one-step ELISAs with no first wash step, excess antigen can bind to detection antibody and result in it washing away, -> falsely low concentration
Explain what is meant by “spectral interference” in a laboratory test.
If the sample is abnormally colored (hemolysis, icterus [jaundice], lipemia), it will give an abnormal reading on a spectrophotometer, leading to an abnormal test result
Define analyte.
the item being measured or determined in a laboratory test
Define what is a “heterophile” or HAMA antibody.
Heterophile antibody: an autoantibody that binds to a self protein, so pt antibody can recognize antibodies used in assay
–HAMA = Human Anti-Mouse Antibodies
Antibodies contained in some patients blood samples may positively or negatively interfere with immunoassay test results (false positives and false negatives)
Define critical tests and critical results.
Critical tests: any test that always requires rapid communication to the ordering provider, whether tests are “normal” or not
–Ex. HIV testing following an accidental needle stick (must know whether it’s positive OR negative)
Critical results: any lab finding that requires a rapid communication of results
- -Anything that is so absurdly high or low the patient has a risk of dying unless action is taken immediately
- -Ex. glucose 500 mg/dl
List several factors that can lead to analytic variation. (3)
Assay imprecision: repeated measurement of the same sample can produce disparate results
Systematic analytic error: values differ between labs because of calibration differences, lack of standardization
Assay or immune interference: spectral interference, Hook effect, HAMA
Pseudohyponatremia: what is it and what causes it?
Reported concentration of sodium in plasma/serum is less than true value
Cause: very high concentrations of serum proteins and lipids may produce a falsely low result for serum sodium
–knowledge of how it happens NOT important but knowledge that it can occur is important
True/false: If the laboratory results for a patient do not match the clinical picture it is important to contact the laboratory performing the test to confer with the Laboratory’s Medical Director.
True. This is the most important takeaway from the lecture.
Describe some of the problems leading to erroneous results that occur in the post-analytic stage of the test. (3)
Transcription errors: placing a decimal in the wrong place, recoding the correct result for the wrong patient
Calculation errors: neglecting to account for a manual dilution error
Errors in result interpretation
