Using Laboratory Data Flashcards
Reasons to order lab test
Diagnosis:confirm, screen, rule-out
Management: monitor, follow course, prognosis
Fear: criticism, insecurity
Most common reasons for ordering a lab test?
diagnosis, screening, monitoring
Lab test most likely to change the diagnosis and direct therapy?
microbiology reports
Reference ranges are based on
range of values determined by large numbers of normal, healthy individuals (may be age, sex, race specific)
Reference range is calculated by
mean value and the standard deviation
Reference range is a ____________ curve
bell-shaped, Gaussian ST of +/-2 to include 95%
Total bilirubin reference range curve
Non-Gaussian
Total serum cholesterol reference range
is a “healthy” rand based on epidemiological studies
Serum creatinine is a good example of
the necessity of comparing current value to previously determined value
Endocrinology tests reference ranges are based on
stimulatory and inhibitory recordings
In disease prognosis/progression it is important to
monitor change sin value (not reference range) i.e.. LDL, PSA
What is the chance of an abnormal result if you do 1 test?
2 ind tests?
5 ind tests?
10 ind tests?
5%
10%
23%
40%
Action levels
abnormal result that ay or may not require treatment, cholesterol > 200, LDL > 160, HDL < 35 –> patient should be more active
Sensitivity
probability that a patient with a disease will test positive
Sensitivity equation
Sn = 100X TP/ (TP + FN)
Specificity
probability that a non-diseased patient will test negative
Specificity equation
Sp = 100X TN / (TN + FP)
Prevalence
pre-test probability of disease in the population
Altering the sensitivity will
alter the specificity of a test and vice versa
Choose a test with HIGH Sensitivity when
the probable disease is serious and curable
Want a very sensitive test when the
benefit of detecting the disease are great
Choose a test with a HIGH specificity when
the probable disease is serious but not treatable
Want a highly specific test when the
risks of wrong diagnosis is great
Confirmatory test
highly sensitive
Common order of testing
sensitive tests followed by specific
PPV
probability that a positive test result indicates disease
PPV equation
PPV= 100X TP/ (TP + FP)
NPV
probability that a negative test result indicates no disease
NPV equation
NPV= 100X TN/ (TN + FN)
Does prevalence effect the predictive value?
Yes, the less prevalent a disease the lower the PPV. Higher prevalence = higher PPV
Does sensitivity and specificity effect predictive value?
Yes, the more sensitive/specific, the higher the PPV.
Serial testing effect on sensitivity
decreases
Serial testing effect on specificity
increases
Parallel testing effect on sensitivity? specificity?
higher sensitivity, lower specificity
What test would you use for a mild treatable disease?
good sensitivity and specificity
Order of testing for clinicians
high sensitivity test followed high by specificity test
Once prevalence reaches >50% what effect does it have on predictive value
not much increase in predictive value
Is predictive value effected by cut-off values
Yes, cut off values change the sensitivity and specificity of a test and therefore change the PPV and NPV
Pre-analytic errors
medication interference, wrong patient prep, inadequate amt of specimen, wrong specimen, delayed transport, wrong storage
Pre-analytic variation
age, gender, mass, prep of patient, posture of patient, specimen type
Analytic errors
wrong usage of equipment, expired reagents
Analytic variables
analytical sensitivity, analytical specificity, analytical interference, precision, accuracy, drug impact, lab instruments
Analytical sensitivity
ability to detect a substance; least concentration measurable by the test (high… PCR)
Analytical specificity
ability to detect only 1 substance;
Precision
reproducible
accuracy
closeness to a known accepted value
Post-analytical errors
wrong results, time delays
Physician Factors influencing decision to order tests
faulty decision-making, over reliance, lack of understanding, lack of appreciation of testing limitations, etc
Laboratory Factors influencing decision to order tests
convenience of automation, rapid, new tests without proven results, etc
Medical Science factors influencing lab tests
new treatments that require monitoring
Patient and Societal factors influencing lab tests
expectations, health ins, etc
Hospital factors influencing lab tests
encouragement, ICU and dialysis centers, decreased length of stay, more severely ill patients
