Understanding Lab Tests

Question 1

Define what is meant by a reference range.

Answer 1

• Reference interval is more correct

* Interval of values that encompass 95% of the population

Question 2

State what percentage of normal individuals fall outside the reference range by definition.

Answer 2

• 5% or 1/20

Question 3

Explain what is meant by pre-analytic, analytic and post-analytic periods relative to laboratory tests.

Answer 3

• Pre-analytical: Errors before the sample arrives at the lab
o Collection
o Storage
o Transport
• Analytical: Errors in how sample is processed in the laboratory
o Receipt
o Analysis
o Result verification
• Post-analytical: Errors in use and interpretation of the result
o Physician knowledge
o Interpretation
o Implementation of treatment course

Question 4

Discuss the relationship between “reference range” and therapeutic objectives relative to laboratory tests for troponin and HbA1c.

Answer 4

•	Troponin
     -	Biomarker for myocyte damage
     -	False-negative rate of 1%
     -	Only detects troponin levels above 99th percentile
     -	Varies by manufacturer!
•	HbAIc
      -	Reference interval for non-diabetics 4.3-6.1%
      -	Target for therapy: 7%

Question 5

Describe some of the problems leading to erroneous results that occur in the pre-analytic stage of the test.

Answer 5

• Order entry
• Patient preparation
• Specimen collection
• Contamination (pathogenic or other)
• Processing Storage & transport

Question 6

Explain what is meant by “spectral interference” in a laboratory test.

Answer 6

• UV analysis for laboratories
• Sample, contamination, assay chemicals, dilution… can all alter spectral result
• All tests must go testing for interference

Question 7

Explain the “Hook effect” in laboratory tests using an antigen-antibody system.

Answer 7

• False negative when there is too much antigen (can’t read fluorescent signal)
• Corrected by diluting the sample

Question 8

Define what is a “heterophile” or HAMA antibody.

Answer 8

• Patient has antibodies that recognize the antibodies used in the assay
• Can result in false positive and negatives
• Heterophile interference is manufacturer specific to send sample to a different laboratory with difference manufacturer