lab: -By hand. -Pneumatic tube. -Car. -Plane. Physician's request goes to the lab ... patient. Every lab report is an answer to your question. Often, the question is.
General concepts: Introduction to the Clinical Laboratory What is a lab test for?
The International Federation of Clinical Chemistry estimates that laboratories participate in about 80% clinical decisions
The physician produces a lab request: - Introduces patients’ data into the computer. - Requests for tests.
Results are available to the physician’s computer
The computer generates a lab order for the sample collection team, with a specific bar-code label and the list of tubes needed: -Total blood. -Serum. -EDTA-plasma. -Heparin-plasma. -Urine, etc.
Tubes are transported to the lab: -By hand -Pneumatic tube -Car -Plane
Physician’s request goes to the lab computer and to the analyzers
The medical lab scientist examines and validates the results Samples are loaded in the machines and automatically analyzed
EDTA as anticoagulant Hematology
Citrate as anticoagulant Coagulation
No anticoagulant Biochemistry
Plasma
Serum
Blood collected without anticoagulant is allowed to clot and centrifuged to obtain SERUM
Blood collected with anticoagulant is centrifuged to obtain PLASMA
• Plasma contains coagulation factors • Most biochemical analyses are done in serum. • Coagulation analyses and erythrocyte sedimentation are done in plasma
Urine containers for random samples
Urine containers for 24-hour collection
Urine containers may include a preservative to inhibit bacterial growth or acid to stabilize certain metabolites
Hemolysis Hyperlipemia
Hemolysis Jaundice
Hemolysis, hyperlipemia or jaundice interfere in many biochemical methods.
These ass**les from the lab are fu**ing slow! FFUUUUUU!
Remember: No matter how busy you are, you have to be sure that your request has readily been sent to the lab!
Excuse me, I sent you an analysis request from patient X, more than one hour ago, and I haven’t received the results yet !!! Mmmmhhh... Really? Let me check it on my computer... Thank you, I’m waiting... Eeeehh, I’m sorry, but we have not received any request from patient X. Are you sure you have send it to us? What? Yes! Well, no... Wait a minute... Ups, the tubes are on my desk... I forgot... Sorry, I shall send them immediately... Thank you, we shall process them, and you’ll have your results in 20 minutes • Thank you, thank you, I am so busy...
How can we have the results if we have not received the samples?
Pneumatic tubes do not always work properly
Laboratory
Clinical Department/Service
A lab request is a question about the patient. Every lab report is an answer to your question.
Often, the question is whether a parameter is “normal” or “abnormal”. Results from your patient are compared with the “reference interval” for that parameter. What does “abnormal” mean?
• By definition, 5% of the healthy people have a result outside the reference interval. This is called a false positive. • The further a result is out from the reference interval, the more likely is to indicate a disease. • Patients with a specific disease can also have a result inside the reference interval. This is called a false negative.
Normal people often have abnormal results !!!
Remember: The more tests you request in a normal person, the higher the probability is you find an abnormal result JUST BY CHANCE.
Diseased people may have normal results !!!
Remember: Sometimes changes within the reference interval may have clinical relevance. For example, elder people use to have serum creatinine values at the lower part of the normal distribution. A raise to the top may suggest a renal failure.
Several factors influence reference intervals
Age
Gender
Country (ethnicity, diet, habits, environment)
Limitation: Most labs obtain reference intervals from: •The scientific literature (populations different to the local). •The indications of the reagents’ manufacturers.
All biological parameters have individual variability. • Interlaboratory. • Interday. • Timing. • Analytical variability.
Analytical variability is estimated by two concepts: • Precision: How reproducible is your assay. • Accuracy: How close the measured value is to the real value.
• Selection of appropriate tests should be based on the probability of the supposed diagnosis. • This probability is based on the patient’s history, physical exploration and anamnesis, and the prevalence of the supposed disease. • Alterations in more than one tests related to a specific disease have a higher probability to be clinically significant than alterations in just one test. • The higher the degree of alteration, the higher the probability of representing a real disease.
Remember: No rule is going to substitute your brain. To think a little bit helps you to ask the lab for the right tests and to obtain useful results
••
False positives
False negatives
A
B
An excellent test. We call it a biomarker.
A
B
Average test. Many tests are like that.
False positives and false negatives define the sensitivity and specificity of a lab test
A This test is useless.
B
Sensitivity: The incidence of altered results in patients. 6/10 diseased people have altered results.
••
Specificity: The incidence of normal results in healthy people. 10/12 normal subjects have normal results.
False positives
False negatives
A
B
We can change sensitivity and specificity modifiying the cutoff point.
Sensitivity: 20% Specificity: 100%
••
Sensitivity: 60% Specificity: 83% Sensitivity: 100% Specificity: 50%
A High sensitivity ⇒ Low specificity (false positives). High specificity ⇒ Low sensitivity (false negatives).
B
Screening tests have cutoffs to opmitize sensitivity despite the presence of a high number of false positives. This is because the risk of misdiagnosing a patient is much higher than the risk of misdiagnosing a normal person. HIV screening test has a very high sensitivity. HIV confirmation tests (Western-Blot, viral load) have a high specificity.
• Sensitivity and specificity are inherent properties of a test, and are useful in describing its expected performance. But they can only be measured if the actual disease status of individuals undergoing the test is known. • Naturally, when we apply a test in normal clinical practice we do not know who has the disease; we are using the test to help find out. Therefore, we are more interested in what a negative or positive test result means for the patient. For this, we use the predictive values. • The positive predictive value (PPV) shows what fraction of patients who receive a positive test result actually have the disease. a/(a+b), or TP/(TP+FP). • The negative predictive value (NPV) shows how many people who receive a negative score really do not have the condition: d/(c+d), or TN/ (TN+FN).
Diagnostic accuracy of one or several tests can be measured by ReceiverOperating Characteristics analysis – ROC PLOTS.
To do a ROC plot you must calculate sensitivity and specificity at different cutoffs. The closer the plot is to 1, the better. ROC plots are readily calculated by the SPSS program.
16
14
12
Values
10
8
6
4
2
0 0
0, 5
1
No
1,5
Yes
Disease
2
2,5
