“We need to run this sample to the laboratory…STAT!”
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Evelyn Lockhart, M.D. Associate Medical Director, Duke Transfusion Service Assistant Professor of Pathology Body and Disease Lecture Series 2011 March 2, 2011
“We need to run this sample to the laboratory…STAT!”
Evelyn Lockhart, M.D. Associate Medical Director, Duke Transfusion Service Assistant Professor of Pathology Body and Disease Lecture Series 2011 March 2, 2011
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1. Laboratory Medicine—what is it?
2. Describe the role of laboratory data in clinical medical practice
3. Review principles of laboratory testing
4. How to collect, label, and transport patient samples
5. Errors in laboratory medicine
6. Case studies
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Most of us will not be pathologists. But we will be using laboratory medicine and anatomical pathology to treat patients. She wants to help us learn how to use and interpret laboratory test to optimize patient care.
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practical info
“We need to run this sample to the laboratory…STAT!”
Anatomic Pathology: disease diagnosis based on gross and microscopic examination of tissues, organs, and whole bodies.
Clinical Pathology (a.k.a. Laboratory Medicine): disease diagnosis based on laboratory analysis of bodily fluids or tissues
These two fields have growing areas of overlap (such as molecular diagnostics)
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Actually examining tissues, organs, etc. Using your eyes to examine patterns and come up with a diagnosis.
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Biochemical changes, enzymatic changes, etc.
1. Provides medical supervision and oversees managerial supervision of all clinical laboratories.
2. Ensures laboratory compliance with local, state, federal and voluntary regulatory agencies
3. Consults with clinicians to guide lab test ordering, interpretation of lab values, and clinical decision making based on lab data.
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Liason of knowledge between the technical aspects of lab medicine and actual clinical practice
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Makes sure we meet the standards of FDA, etc and are "kid tested, mother approved"
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There are 1,000s of tests, we cannot possibly know all about every test. Clinical pathologists are here to help you!
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Big picture of all the areas of lab medicine that clinical pathologists are concerned with impacting, in terms of proper execution and identification of errors.
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All these steps occur with every Glucose, Hematocrit, etc you order
Survey of 100 M.D.s at Mass General Hospital
Evaluate impact of pathologist interpretation on coagulation test panel:
78% : impacted differential diagnosis
75%: saved physician time
43%: reduced time to diagnosis
30%: reduced lab testing
18%: reduced medical procedures
Data from M. Laposata, 2004
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with 100% response rate!
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Important!
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These are very real, important impacts.
Why do we order laboratory tests?
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see the next slide
To confirm a clinical impression
To rule out a diagnosis
To monitor therapy/disease course
To establish prognosis
To screen for disease
To prevent liability
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Read these
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The flavor of disease
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This should not your key reason for ordering a test.
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monitor certain biochemical markers
Chemistry
Microbiology/immunology
Serology
Hematology/coagulation
Blood bank/Transfusion Service
Molecular diagnostics/proteomics
Toxicology
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The tools we have. She will go through each of these one by one.
Analyzes chemical components of blood, serum, urine, body fluids
Example tests: Electrolytes Tissue enzymes Blood gas Hormones Tumor markers Urinalysis
Urate crystals in urine
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Na, K, Cl
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like troponin, creatine phosphate released from damaged heart muscle
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Demonstrates the overlap between anatomical and clinical path techniques. In a clinical lab you'd still use a microscope to examine the urine speicimen and see the crystals.
Cultures/identifies infectious organisms and the body’s immune response to them
Common tests: Bacterial, fungal, and viral cultures
Antibiotic sensitivity or organisms
Serologic immune response
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"I won't belabor that anymore because I know you all just had a good dose of that'
Analyzes response to infection, tumor, non-neoplastic disease, immunization
Common tests:
Infectious organism antibody titers
Autoantibodies
tumor markers
Anti-nuclear antibodies in systemic lupus
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Looking at different Antibody levels.
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using abo to follow levels of certain proteins
Analysis of genetic material, including single genes and chromosomes from both humans and pathogens
Common tests:
Infectious organism identification
Chromosomal abnormalities
Prognostic indicators
Forensic applications
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A great ex of the overlap between anatomical and clinical path. Source of genetic material could be from blood, urine or from tissue (like a tumor)
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Note: CSI is not always an accurate picture of reality...
Analyzes toxins, drug levels, environmental/therapeutic insults
Common tests: heavy metal poisons, antibiotics, chemotherapeutic agents, anti-convulsive agents
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therapeutic drug range-what is an expected, safe level of drug to be in the patient's blood during the course of treatment.
Provides blood products for patients with hematologic disorders/surgery/bleeding.
Consults on patients with transfusion issues (component selection, transfusion reaction, etc).
May be responsible for blood collection and blood product manufacturing.
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compatibility testing for blood matching; if patient has an adverse reaction-what went wrong and why
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Only called a blood bank if you collect the blood. We get our blood supply from American Red Cross, se we are solely a transfusion service.
Analyzes blood and bone marrow, focusing on cellular elements and coagulation factors.
We don't just do the routine hematology markers-platelet, CBC, etc. We also look at malignant heme as well-like from a bone marrow biopsy
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Also identify coagulation disorders as well-a very complicated/confusing test panel and results. Consult with a pathologist
Duke annual test volume: 9.6 million
70% of all clinical decisions are based on laboratory data
Laboratory medicine accounts for only 3.5% of total health care costs
*Institute of Medicine data
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We get the right diagnosis almost every single time.
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tiny cost
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Huge percentage
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Think of pathology as the business of information and data generation. We must make sure the information we get is correct and that the message is communicated clearly and interpreted correctly
“What does it all mean, Basil?”
The central question:
“Is my patient healthy, or do they have a disease?”
Diagnostic Discrimination
of the test
Test Variability
Variability of Diseased population
Variability of Healthy population
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How well will the test give me the answer to my question.
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The test may be very accurate or it may be like asking an 8 ball (that would be a bad test...)
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The range of normal
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Can the disease look different in different people
Precision: the ability of a test to produce nearly identical values when repeated under identical conditions.
A.k.a. repeatability, reproducibility
Accuracy: the ability of a test to produce results
that are close to the “true” measurement.
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HIGH YIELD SLIDE-ON BOARDS!
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If you repeat the exact same test, do you get the exact same result?
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I KNOW there are actually 3.5 g of substrate A in this tube. How close does my test get to giving the actual measurement of 3.5 g?
All in same area (repeatability) Not close to bullseye (poor accuracy)
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What we want our tests to be
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All over the place (not precise/repeatable), Avg of values is actually very close to bullseye (accurate)
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This is the magic 8 ball. not a good test
People, with or without diseases, show physiologic and biochemical variability.
Defining a range of normal (a.k.a. reference range) is an effort to quantify lab values in individuals that are disease-free.
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Actually can be very challenging. But it is critical to define normal in order to be able to define abnormal (or diseased)
Analyte “X” quantity
1 100
Disease- Free
With Disease
Su
bje
cts
Test
ed
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No overlap. Crystal clear difference...this is NOT reality
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Analyte “X” quantity
1 100
Disease- Free
With Disease
Su
bje
cts
Test
ed
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Both are ranges, and there may be overlap
A range of acceptable values for an analyte based on a healthy cohort.
Classically determined from samples available to a laboratory.
Age, sex, exercise, diet—multiple impacting factors on lab values
A priori criteria to determine “healthy” individuals
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Ex. if want to establish normal values for a clotting test, you would need to screen people before putting them in your 'healthy' subjects group. Ask if they have heavy menses, bleed a lot, easy bruising, etc.
You have a 25 year old male patient you suspect as having an acute bacterial infection
You order a white blood cell (WBC) count as part of your evaluation
Result? WBC = 15,000/mL
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Is this elevation of WBC (Leukocytosis)?
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variability, but most people between 6 and 7. The guy on the previous slide with 15 is off the charts high
Data elements centered around their mean in a “bell-shaped” pattern. Mean: sum of all results divided by number of results
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A normal distribution. even and symmetrically distributed around a mean
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-3SD: the value that is 3 standard deviations below the mean
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68.2% of ppl will fall within 1 SD below and 1 SD above the mean
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95.5% of ppl will fall within the interval bound by 2 SD below and 2 SD above the mean
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99.7% of ppl will fall within the interval bound by 3 SD below and 3 SD above the mean
Standard deviation: value describing typical difference from the mean. Defined by the sum of squared differences from mean divided by N-1. Reference ranges often defined for values in a normal
distribution by +/- 2 S.D.
Coefficient of variation: Standard distribution of a set divided by the mean of a set. Expressed as a percentage. E.g.: if the mean is 50, and the SD is 3, the C.V. is 6%
(3/50 x 100)
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As per the graph on the previous page, this will include 95.5% of people. Thus, a reference range WILL NOT CAPTURE ALL NORMAL VALUES!!!
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C.V.=SD/Mean
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An important point: reference/normal values are defined for a Specific Population. What is used in a Minneapolis lab should not necessarily be used in Mexico.
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You're not going to be tested on these.
Any lab test is compared to a criterion standard (“gold standard”) for defining disease
Gold standard Lab Test
Autopsy showing myocardial ischemia and necrosis
Serum cardiac enzymes
Prostate biopsy showing prostatic adenocarcinoma
Serum prostate specific antigen
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M.I.
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The Best Way we know to test for a specific disease. All new tests must be compared to this test. We want to know if the lab tests on the right are as good as the gold standard on the left. She pointed out that we don't really know if the gold standard is perfect, but it's just the best "mouse trap" we have right now. If a better test is discovered, it will become the new gold standard.
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Prostate Cancer
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Actual disease
“GOLD STANDARD”
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HIGH YIELD!!!!
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Assumed to be truth
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TP: Test pos, patient has disease FP: Test pos, patient does not have disease - FN: Test neg, but patient has disease (test missed it) TN: Test neg, patient does not have disease
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Which is worse? FN or FP? Hard to say FP could lead to dangerous, unnecessary treatments. FN could lead to lack of necessary treatment or timely intervention.
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Clinical Pathologists can move the threshold level that indicated disease in order to minimize the number of FP/FN results.
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Ex 1. threshold cut-off around 50.
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no disease, but above current threshold=FP
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Disease, but below current threshold=FN
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Want to increase specificity-make sure a positive value meant they really have disease (ie get rid of FP). Move threshold up so that high range of normal is no longer considered disease. You will fail to i.d. some with disease though (FN)
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Want to increase sensitivity: Do not want to miss a single person with disease-we can move threshold down. But we will increase false positive rate because people on the high end of normal will be identified as diseased.
True positive ______________________
True positive + false negative
The probability of getting a positive result in a diseased patient
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All patients that have disease
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Patients who have disease and tested positive
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A measure of the test itself
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This is a division line
True negative _________________
True negative + false positive
The probability of getting a negative result in a disease-free patient
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Total number of disease-free patients
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dividing bar
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people without disease who tested negative
True positive _________________
True positive + false positive
The probability of having a disease, given a positive test result
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This is what the patients care about. If this test says I have HIV, what is the chance that I actually have it?
True negative ___________________
True negative + false negative
The probability of being disease-free, given a negative test result
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If this test says I am HIV-negative, how sure can I be that I really don't have HIV?
The predictive value of a test is dependent on two things:
1. The accuracy of the test
2. The prevalence of the disease in the population
Prevalence: The percentage of persons with a given disease within a given population at a given time
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This VERY STRONGLY impacts the predictive value of a test
A patient comes in to your office having done a home HIV test which is positive. She wants to know what the chances are that she is actually infected.
She is “low risk” for having HIV (in a population with a prevalence of HIV positivity of 1 in 100,000).
The test kit claims 99% sensitivity and 99% specificity
Spitalnik, Hospital Physician, Sept, 2004
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low prevalence
HIV+
Disease
HIV-
Disease
Total
HIV + Test 99 (TP) 99,999 (FP) 100,908
HIV – Test 1 (FN) 9,899,901
(TN)
9,899,902
Total 100 9,999,900 10,000,000
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We have to use such large numbers because the prevalence is so rare
Positive predictive value (TP/TP +FP) = 99/100,098 = 0.1%
She has 0.1% of actually being positive
Even with high sensitivity and specificity, prevalence has a tremendous impact on result interpretation
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Confirmation of a disease by laboratory testing will have a higher PPV in a group pre-screened for the disease.
Why? Because a group that is pre-screened has a
higher prevalence of the disease than the general population.
Example: screening test for HIV infection is an antibody test;
confirmatory would be a test panel including molecular test for viral RNA.
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Screening Test: Very High sensitivity, you may pick up lots of false positives. But this creates population for the actual confirmatory test that has a higher prevalence of the disease than the initial/general pop. This increases the PPV of the confirmatory test.
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The computer lab information system we have for communicating test results to physicina.s
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Bolded red result=abnormally low
Lab values as such great variance from normal that it represents a life-threatening pathophysiologic state.
Must be communicated immediately to a licensed health care professional
Recommend readback of patient name, medical record number, and critical value.
Examples:
Hemoglobin < 5.0 g/dL
Potassium < 2.8 or > 6.0 mmol/L
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Laboratory will call a nurse, physician directly. You should take this call.
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to ensure comprehension and correct understanding of info
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http://www.youtube.com/watch?v=gh5xu35bAxA She showed a video to demonstrate that it can be difficult to communicate over the telephone, may have different accent, etc. you must be careful.
