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Clinical Lab: Intro


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Benefits of Diagnostic Tests
-disease screening (identify risk factors)
-patient management (severity of dz, prognosis, monitor disease progress)
Negative Side of Laboratory Tests
-managed care pre-approval/justification
-morbidity/mortality associated with tests (false positive or false negatives)
-laboratory inequalities
-time factors (minutes to days)
Cost of health care
-over 1.3 trillion spent on health care in U.S.
-lab tests comprise approx 10% ot total health care costs
Pre-ordering quetions
-how will the test help you?
-why are you ordering the test?
-will the test results change your plans?
-what do you hope to gain from the test?
-so the risks of testing outweight the benefits of the test results?
-how soon do you need the inoformation?
What makes a test clinically useful?
-methodology is well described and clinical useful
-test is accurate and precise
-test has well established reference ranges
-test is sensitive and specific
-does the test give you the correct answer
-does it measure what it is supposed to measure
Sources of inaccuracy
-Human error (obtaining the sample, performance of the test, transciption of the results, interpretation of the results)
-wrong patient
-broken equipment
-to what detail can the test detect differences

-e.g. measuring the height of a tree
56 steps
20.5 yards
62 feet 3 inches
62 feet 3.247638349 inches
-likelihood that you will get the same result next time you do the test

-results usually fall within a Gaussian distribution (bell curve)
-normal range is +/- 2 standard deviations (95%)
-the more precise a test, the more evident the error
-ability of a test to detect patient with a specific disease
-amount of false negatives results
-ability to detect only the analyte of interest
-how well test abnormality is restricted to those with the disease in question
-amount of false positive results
True Positives
-test says positive
-real answer is positive
False Positives
-test says positive
-real answer is negative
True Negatives
-test says negative
-real asnwer is negative
False negatives
-test says negative
-real answer is positive
Sensitivity calculation
TP/ (TP + FN) x 100

ability of test to detect a (+) result
Specificity calculation
TN/ (TN + FP) x 100

ability of the test to be correct about + result
-how often the condition occurs in nature

can influence the predictive value
Predictive Value
-smaller the number of cases of the disease in the population, the more likely the (+) test result will be a false positive

-the more common the disease in the population, the more likely that the (+) result is true
Predictive Value Calculation
TP/ (TP + FP) x 100
Reference Ranges
-normal population has lab values which fall within a gaussian distribution
-95% of the group lie within 2 standard deviation
-many ranges are age dependent
-normal range may very between labs
Screening Tests should be..
-high prevalance of disease in population
-signigicant morbidity and mortality
-effective tx available
-improved outcome if early intervention
Screening tool for colon cancer
-high sensitivity
-low specificity
-low cost
Mammogram for Breast Cancer
-moderate to low sensitivity
-high specificity
-moderate cost
Brain Biopsy screening tool for tumor
-high specificity
-low sensitivity
-high cost
-not very innocuous
Three cateogories for laboratory errors
1. pre-analytical
2. analytical
3. post-analytical
Pre-analytical Errors occurs:
-at time of pt assessment
-test order entry
-request completion
-communication to lab
-specimen collection
-specimen transoport
-specimen receipt in lab
Preanalytical Errors in specimen collection
-sample too small
-sample contominated
-sample lost
-sample mixed up with others
-sample mis-labeled
-sample collected in wrong tube
Preanalytical Errors in improper pt preparation
-collection site not prepared
-fasting state
-time of day
Preanalytical Errors in Environmental Factors
-sample got hot/cold
-sample was shaken
-sample was exposed to light
Preanalytical Errors in Timing
-different tests need to by run within different time periods
Preanalytical error: Common conditions that effect lab results
-IV fluids running at time of collection
-effect of dehydration
-effect of heparin flushes (sample collected from ports or IV lines)
-administration od meds at unexpected times
-poor communication between provider and nurses
Analytical phase beginning and end
-begins when specimen is prepped for testing
-ends when result is interpreted and results are verified and ready to report
Sources of analytic error include:
-test to test carry over
-improper processing
-interfering substances
-instrument failure
Post-analytical error
-results are released to clinician
-clinician interprets results
-clinician makes diagnostic or therapeutic deceisions
-means now
-lab drops it routine work-laod to comply
-generally available eithin 1 hour of request
-next available run or when tech is available
-will be done when most efficient
-usually within 24 hours
Ancillary Testing: point of care
-no central clinical lab on site but some tests can be done in the oddice or on pt floor by trained persons
-CLIA waived test
Examples of ancillary testing
-fingerstick glucose
-urine dipstick
-fecal occult blood
-pregnancy testing
-spun hematocrit
-ovulation test
-Physician performed microscopy
-Clinical Laboratory Improvement Amendments of 1988
-set standards to improve quality in all laboratory services
-US department of health and human services
CLIA categorized lab test
-PPM (provider-performed microscopy procedures)
-moderate complexity
-high complexity
-performed by physician, mid-level practitioner, dentist
-moderately complex procedure
-primary instrument is microscope
-limited specimen handling required
PPM includes:
-wet preps and KOH
-pinworm exam
-urine sediment exam

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