S1B5 - Intro to Clinical Lab

Question 1

What is the sequence of events involved between ordering laboratory tests and getting the results?

Answer 1

1. You (the doctor) write an order in the chart or clinic encounter sheet, or enter it in the computer.
2. In most instances a clinic or floor employee will transcribe this order either through the computer or on a requisition
3. This order is sent to the lab, and a phlebotomist or technologist is sent to draw the test. In certain units, though, medical or nursing staff may obtain the specimen. In some places, the medical students and residents are used as phlebotomists after hours. (Being slave labor adds much to the learning experience!) Either way, the specimen is collected
4. The specimen is labeled at the bedside and sent to the lab
5. It is accessioned in the lab, and the name on the tube is compared to that on the requisition/order, as well as the test(s) ordered and the appropriateness of the specimen
6. The specimens are distributed to the appropriate section for testing
7. The lab runs the appropriate test(s) on the appropriate instrument(s).
8. Results are generated, reviewed by the technologist, and reported to the appropriate patient care area. Critical values are repeated for verification before being called by to the nurse or physician
9. A hard copy of the report is sent to the physician or nursing unit, and an electronic copy is entered into the electronic medical record.
10. The physician then uses the results to make clinical decisions for the patient

Question 2

Variables that affect the validity of laboratory tests fall into what three categories?

Answer 2

Variables that affect the validity of laboratory tests fall into three categories: 

• Preanalytical 
• Analytical 
• Post-analytical

Question 3

What are the three categories of preanalytical variables that can affect the validity of test results?

Answer 3

Preanalytical variables are those that affect test results BEFORE they are analyzed. They may be subclassified into three categories:

• Sample collection
• Sample handling
• Patient factors

Question 4

What are the two main categories of analytical variables that can interfere with the viability of lab results?

Answer 4

Analytical Variables: This is anything that can interfere with the actual analysis of the test by the lab. It includes, but is not limited to:

• Interfering substances in the sample that affect the methodology of the test
  
  • Drugs
  • Anticoagulants
  • Contaminants
  • Antibodies
  • Radiology contrast media
  • Lipemia
  • Hemolysis
  • Unknown agents
• Instrument issues
  
  • Mechanical problems (tubing, probes, cuvettes, optics, etc.)
  • Reagent problems
  • Calibration problems
  • Quality control problems
  • Climate control problems (too hot or too cold in the lab)

Question 5

What are post-analytical variables?

Answer 5

A post-analytical variable is anything that can happen after the results are reported. This may include:

• Interpretation of the result in light of clinical findings (Example: Patient has an elevated alkaline phosphatase. Is it because he has cancer with bony metastases, or because he has an obstructed gallbladder?) 
• Physician does not receive a report because his/her name was not on the requisition as the ordering physician. 
• Manual data transfer in the medical record (Example: “Hepatitis” gets put in clinic chart after lab results return, but type is not specified by the writer.) 
• Verbal or written miscommunication between healthcare personnel, particularly when patient is being transferred to another floor or institution.
• Physician treating the numbers and not the patient, without considering false positive/false negative test result.

Question 6

What things should you be sure that you have accurate results of before you make a major decision about them?

Answer 6

NEVER make a major decision about potassium supplementation, blood product transfusion, antibiotics, or surgery until you have ensured that the result is accurate.

Question 7

What is the two-step postanalytic review that laboratory results undergo?

Answer 7

Laboratory results undergo a two-step postanalytic review:

• for analytic correctness, by the technologist (using delta checks, linearity ranges, etc.),
• and for clinical significance, by the physician (applying critical values, reference ranges, pre-test and post-test probability, etc.).

Question 8

What are reference intervals most commonly defined as?

Answer 8

Reference intervals (normal ranges) are most commonly defined as the range of values into which 95% of people without disease will fall. By this definition, 5% of individuals (1 in 20) with no disease can have laboratory results outside the reference (normal) range.

Question 9

What does test sensitivity mean? What is the focus of sensitivity?

Answer 9

• Sensitivity is the probability of a positive result in a person with the disease (true positive rate).
• The focus of sensitivity is on individuals with the disease.
• The fewer false negatives, the higher the sensitivity.
• The more sensitive a test is, the more effectively it correctly identifies diseased individuals.
• The formula is based on looking at all the groups with disease.

Question 10

What does test specificity mean? What is the focus of test specificity?

Answer 10

• Specificity is the probability of a negative result in a person without disease (true negative rate).
• The focus of specificity is on individuals without the disease, and the formula for specificity is based on looking at all the groups without disease.
• The fewer false positives, the higher the specificity.

Question 11

Predictive Value of a Positive Test:

• what is the focus?
• what is it greatly influenced by?
• how to you calculate it?
• what occurs more often when prevalence is low?

Answer 11

Predictive Value of a Positive Test

• Focus is on the population with a positive test, regardless of disease status.
• The better the positive predictive value, the more likely a positive test means the individual has the disease.
• This is greatly influenced by prevalence of the disease, i.e., positive HIV tests are more likely valid in areas where HIV is more prevalent.
• Many more false positives than true positives occur when the prevalence is low.

Question 12

Predictive Value of a Negative Test:

• what is the focus on?
• what influences it?
• how do you calculate it?

Answer 12

Predictive Value of a Negative Test

• Focus is on the population with a negative test, regardless of disease status.
• The better the negative predictive value, the more likely a negative test means the individual does not have the disease.
• Negative predictive value is not greatly influenced by prevalence of disease, since true positives are not part of the equation.

Question 13

What is the difference between prevalence and incidence?

Answer 13

Prevalence vs. Incidence

• Prevalence = # of existing cases in population—often expressed as a percentage of the population 
• Incidence = # of new cases in population/unit time (generally per year)—usually expressed as a number

Question 14

What are key differences between screening tests and confirmatory tests?

Answer 14

Screening Tests versus Confirmatory Tests 

• Screening tests
  
  • Require high sensitivity to minimize the number of cases missed.
  • Are typically for diseases that have effective treatments.
  • The disease must have a high enough prevalence to justify the expense. 
• Confirmatory tests
  
  • Require high specificity to be confident in the diagnosis (i.e., few false positives).