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Immunology

Interpretation

Interpreting a test result is a multi-layered process. It starts with a simple “positive” or “negative” but quickly expands to consider the method, the strength of the result, and how it fits into the broader clinical picture

Qualitative Result: The “Yes or No” Answer

This is the most basic level of interpretation. Is the analyte we’re looking for present or absent?

  • Positive (or Reactive): The test has detected the target antigen or antibody above the established cut-off threshold
  • Negative (or Nonreactive): The test has not detected the target analyte
  • Equivocal (or Indeterminate): The test result falls into a “gray zone” that is neither definitively positive nor negative. This is a frustrating but important result. It often necessitates re-testing the patient with a new sample in a few weeks to see if the result clarifies

Critical Context: The Serological Window Period

A negative result is not always a true negative. The most important reason for a falsely negative result in infectious disease serology is the window period. This is the time between initial infection and the development of a detectable antibody response

  • Example: A patient with early, localized Lyme disease may present with the classic bullseye rash but have a negative antibody test. The diagnosis in this stage is purely clinical. The test is negative not because the patient isn’t infected, but because their immune system hasn’t had time to produce the antibodies we’re looking for

The Semi-Quantitative Result: The Power of the Titer

For many classic serological tests, a simple “positive” is not enough. We need to know how much antibody is present. This is where the titer comes in

  • Definition: A titer is the reciprocal of the last serial dilution of a patient’s serum that still produces a positive test result
  • The Concept: It is a semi-quantitative measure of antibody concentration. A titer of 1:640 indicates a much higher concentration of antibody than a titer of 1:40
  • Interpretation
    • Baseline vs. Convalescent Titers: The real power of the titer is in tracking the course of an infection. A single titer is just a snapshot. To diagnose an acute infection, we look for seroconversion (going from negative to positive) or a four-fold or greater rise in titer between an acute sample (taken when the patient is sick) and a convalescent sample (taken 2-4 weeks later). A jump from 1:16 to 1:64 is a four-fold rise and is diagnostic of a recent infection
    • Monitoring Treatment: For syphilis, the RPR titer is the primary tool for monitoring therapy. A four-fold drop in titer after treatment is the benchmark for a successful cure
    • Interpreting Autoantibodies: For an ANA test, a higher titer (e.g., >1:320) is more likely to be clinically significant and associated with autoimmune disease than a low titer (e.g., 1:80), which can be seen in healthy individuals

Quantitative Result: The Precision of Automation

Modern automated immunoassays provide fully quantitative results, reported in specific units like IU/mL, ng/mL, or copies/mL

  • The Concept: The instrument measures a signal (e.g., light from a chemiluminescent reaction) and compares it to a standard curve that was generated using calibrators with known concentrations of the analyte
  • Interpretation: The result is compared to an established reference interval or cut-off value
    • HIV Viral Load: The result is given in copies/mL. A result of “undetectable” (e.g., <20 copies/mL) is the goal of therapy. A result of 500,000 copies/mL indicates high viral replication
    • Rheumatoid Factor: The result is given in IU/mL. A result of 8 IU/mL might be considered negative, while a result of 150 IU/mL would be strongly positive

Sensitivity and Specificity: The Test’s Performance Characteristics

This is the bedrock of test interpretation. You must understand the inherent strengths and weaknesses of the test you are performing

  • Sensitivity: The ability of a test to correctly identify individuals who have the disease (a measure of true positives)
    • A highly sensitive test is a good screening test because it is excellent at ruling out a disease. A negative result in a sensitive test is very reliable
    • Example: The ANA test by IFA is highly sensitive for SLE. A negative ANA makes a diagnosis of lupus very unlikely
  • Specificity: The ability of a test to correctly identify individuals who do not have the disease (a measure of true negatives)
    • A highly specific test is a good confirmatory test because it is excellent at ruling in a disease. A positive result in a specific test is very reliable
    • Example: The Anti-Sm antibody test is highly specific for SLE. A positive Anti-Sm is virtually diagnostic of the disease. The RPR test, in contrast, has poor specificity due to its many biological false positives

The Pattern: Clues from the Image

Some tests provide more than just a number; they provide a picture that offers valuable diagnostic clues

  • ANA Patterns: The interpretation of an ANA test is incomplete without a description of the staining pattern (homogeneous, speckled, nucleolar, etc.). A homogeneous pattern points the clinician toward ordering an anti-dsDNA test, while a centromere pattern strongly suggests CREST syndrome. The pattern guides the next step in the diagnostic workup
  • Western Blot Bands: The interpretation of a Lyme disease Western Blot is entirely dependent on which specific protein bands are present. A positive IgG blot requires the presence of 5 of the 10 specific bands

Putting It All Together: The Algorithm

In modern diagnostics, we rarely rely on a single test result. We use testing algorithms—a logical sequence of tests designed to maximize accuracy and cost-effectiveness

  • Screening followed by Confirmation: This is the classic approach
    • Syphilis (Traditional): Screen with a sensitive, inexpensive RPR (nontreponemal). If positive, confirm with a highly specific TP-PA (treponemal)
    • Lyme Disease: Screen with a sensitive EIA. If positive, confirm with a highly specific Western Blot
  • The Final Report: A well-constructed laboratory report doesn’t just give a number. It provides context. It might include the method used, the reference range, and often, an interpretive comment written by the lab director or senior technologist. For example, a complex syphilis report might state: “Reactive treponemal test with a nonreactive RPR. This pattern may be seen in successfully treated syphilis or in late latent disease. Clinical correlation is required.” This expert interpretation is an invaluable service to the clinician and the patient