How Laboratory Results Below Detection Limits Are Determined and Reported
When a lab reports a result as "<5" in a range of 0-6, this indicates the analyte was detected but fell below the assay's lower limit of quantitation (LoQ), meaning the exact concentration cannot be reliably measured with acceptable precision, even though some signal was detected.
Understanding Detection Limits
Laboratory assays have three critical thresholds that explain why results are reported as "<5" rather than exact values:
Limit of Blank (LoB): The highest apparent concentration expected when testing a sample containing no analyte, calculated as mean(blank) + 1.645(SD of blank) 1
Limit of Detection (LoD): The lowest concentration that can be reliably distinguished from the blank, calculated as LoB + 1.645(SD of low concentration sample) 1
Limit of Quantitation (LoQ): The lowest concentration where the analyte can be reliably detected AND measured with predefined acceptable precision and bias 1
Why Results Are Not Exact Below the LoQ
The fundamental issue is analytical imprecision at low concentrations:
When measurements fall below the LoQ, the coefficient of variation (CV) typically exceeds 10-20%, making the exact numerical value unreliable 2
Instrument "noise" and measurement error become proportionally larger relative to the true signal at very low concentrations 3
Samples with results below the lower limit of sensitivity must be repeated to rule out technical errors (such as sampling probe errors) and to confirm the value 2
Clinical Laboratory Practice
Guidelines specify how laboratories should handle these low-level results:
Results falling below the lower limit of sensitivity due to sampling errors are uncommon and can be resolved by repeat testing 2
Laboratories should verify by repeat testing specimens with results below the lower limit of the reference interval 2
If confirmed on repeat testing, the laboratory should notify the ordering clinician, as values below the reference range may indicate specific clinical conditions 2
Many laboratories use confirmatory testing for samples with high coefficients of variation between replicate values (generally >10%) to confirm the value 2
Reporting Conventions
The "<5" notation serves specific purposes:
It indicates detection of the analyte but acknowledges that quantitation below this threshold lacks sufficient precision for clinical decision-making 1
This approach avoids false precision—reporting an exact number (e.g., "3.2") when the measurement uncertainty is large would be misleading 3
The practice focuses on avoiding false positives while acknowledging that measurements exist below the threshold 3
Clinical Implications
For molecular diagnostics specifically:
When multiple trackable sequences are available, the assessment for each sequence should be evaluated in the context of their individual limit of detection and uniqueness 2
Low-level results below the LoD require careful interpretation to distinguish clinically significant findings from background noise 2
Repeat testing to evaluate dynamics of the result may be helpful in distinguishing true positive findings from clinically insignificant background 2
Common Pitfalls
Treating all values below the LoQ as equivalent to zero is inappropriate—some signal was detected 3
Automated procedures that apply standard thresholds without considering the measurement process may fail to capture clinically relevant information 3
The lower limit of sensitivity can vary significantly between different assay platforms and methodologies, ranging from 5-20 U/dL depending on the analyte and method 4
Assays with CV >20% at the decision threshold may lead to challenges in clinical interpretation 2