How do I interpret my lab results?

General Principles for Interpreting Laboratory Results

Laboratory test results must always be interpreted within the complete clinical context of the patient's presentation, medication history, and overall clinical picture—never in isolation. 1

Essential Steps Before Interpretation

Obtain Complete Clinical Information

• Document all medications including prescription drugs, over-the-counter medications, and supplements before interpreting any test result 2, 3
• Review the patient's complete medical history, as many conditions and medications can affect test results 1
• Understand the specific clinical question being asked and whether the test result will actually impact management decisions 3

Verify Test Quality and Methodology

• Check specimen validity: Confirm proper collection, handling, and storage conditions 1
• Verify the assay method used, as different methodologies have different sensitivities, specificities, and potential interferences 1
• Review the laboratory's specific cutoff values and reference ranges, which are assay-dependent 1
• Ensure results are reported with appropriate units, methodology, and performing laboratory documentation 1

Understanding Test Limitations

Recognize Analytical Variability

• Small changes in quantitative results (e.g., ±0.3% for HbA1c) may reflect assay variability rather than true clinical change 1
• Screening immunoassays are presumptive only and have inherent limitations in specificity due to cross-reactivity with structurally similar compounds 2, 3, 4
• Results outside expected ranges or below reference intervals should be verified by repeat testing, ideally using a different analytical method 1

Account for Timing Issues

• Test results depend on disease activity, with concentrations varying during acute episodes versus remission 1
• Treatment status affects results, with lower concentrations during immunosuppression or after plasma exchange 1
• Detection windows vary by substance and testing matrix, potentially missing recent or remote exposures 1, 2

Interpreting Positive Results

Confirm Unexpected Positives

• Always review positive results with the patient first to determine whether something other than the suspected condition may explain the results 1
• Consider false-positives due to cross-reactivity, medication interference, or laboratory error 1, 2, 3
• Request confirmatory testing using gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-tandem mass spectrometry (LC-MS/MS) when results are unexpected or will impact patient management 2, 3, 4
• Use a second methodologically different assay if discrepancies exist to reduce false-positive reproduction 1

Assess Clinical Significance

• Interpret results according to local cutoff values and clinical context, not just whether the result is "positive" or "negative" 1
• For antibody testing, repeat positive tests at least 12 weeks apart to rule out transient positivity that is insufficient for diagnosis 1
• Consider the positive predictive value, which varies with disease prevalence in the tested population 5

Interpreting Negative Results

Don't Dismiss Clinical Suspicion

• A negative test does not exclude the possibility of disease, especially if clinical suspicion remains high 1
• Consider retesting during acute episodes, treatment-free intervals, or after appropriate washout periods if initial testing is negative but suspicion persists 1
• Recognize that standard panels may not detect all relevant substances or variants 1, 2

Evaluate for Technical Issues

• Assess whether the specimen could have been diluted, substituted, or adulterated 1
• Consider whether the substance used is included in the test panel 1, 2
• Verify that timing of specimen collection was appropriate for the detection window 1, 2

Critical Pitfalls to Avoid

Never Make Consequential Decisions Without Confirmation

• Do not discharge patients from care, make custody decisions, or take legal actions based solely on screening test results without confirmatory testing 2, 3, 4
• Avoid assuming all positive screening results represent the suspected condition without considering alternative explanations 1, 2, 3
• Never dismiss ongoing symptoms just because a test result is negative 1

Maintain Appropriate Clinical Relationships

• Establish a working relationship with laboratory personnel or toxicologists for consultation on complex or unexpected results 2, 3, 4
• Seek expert advice from specialized centers when results conflict with clinical presentation 1
• Ensure close interaction between laboratory and clinician for proper interpretation 1

Understand Specific Test Characteristics

• Know which medications and conditions can interfere with specific assays 1, 2
• Recognize that different immunoglobulin classes (IgG, IgM, IgA) have different clinical relevance, with IgG generally more clinically significant 1
• Be aware that some tests (like certain antibody assays) may be affected by recent treatments such as IVIG or plasma exchange 1

Documentation and Reporting

Essential Elements to Document

• Immunoglobulin class detected, assay type, antigenic substrate, and biomaterial used 1
• Titer/concentration/units with assay-specific cutoffs 1
• Performing laboratory and methodology 1
• Any potential interferences such as anticoagulant therapy or acute phase proteins 1

Communicate Limitations Clearly

• Report results with warnings about potential interferences and limitations 1
• Provide context about what the test does and does not measure 2, 3
• Include guidance on when repeat or confirmatory testing is indicated 1