TECHNOLOGY

When performing ELISA assays, many researchers encounter a common issue: the optical density (OD) values of some samples are excessively high and exceed the maximum OD value of the standard curve. Some researchers simply discard the data and reload samples; others dilute samples arbitrarily and calculate concentrations directly using the original standard curve. There are also those who keep adjusting dilution factors, resulting in messy data and invalid inter-group comparisons.
This problem is particularly problematic for submissions to SCI journals, methodological validation and large-scale sample testing. Improper handling of out-of-range samples will not only lead to distorted data but also raise questions from reviewers about the rigor of your experiments, delaying manuscript submission and research progress. This guide elaborates on result judgment, serial dilution, retesting and data correction to fully resolve the issue of out-of-range samples.

Two Types of Out-of-Range Results & Corresponding Handling Methods
1. High Concentration Out-of-Range (Most Common)
The sample OD value is higher than the maximum OD value of the standard curve, and the sample concentration exceeds the upper limit of quantitation (ULOQ) of the kit.
• Manifestations: Abnormally high readings or failure of the analysis software to calculate concentrations.
• Cause: Excessively high levels of the target analyte in samples.
2. Low Concentration Out-of-Range
The sample OD value is lower than the minimum OD value of the standard curve, and the sample concentration is below the lower limit of quantitation (LLOQ) of the kit.
• Manifestations: OD values close to blank control wells, and calculated concentrations below the limit of detection.
• Causes: Extremely low content of the target analyte, sample degradation or matrix inhibition.

Core Principles: For high-concentration samples, perform serial dilution and retesting first. Do not concentrate low-concentration samples blindly; instead, assess sample validity prior to further operations.

Handling for Samples Exceeding the Upper Limit of Quantitation (ULOQ)
Step 1: Determine a Proper Dilution Factor (Avoid Arbitrary Dilution)
Many researchers dilute samples 10-fold or 100-fold based on experience in formal tests, which may easily cause dilution bias and aggravate matrix effects.
1.Conduct preliminary tests first
Dilute out-of-range samples with the dedicated diluent for standards and samples provided by the kit. Never use pure water or PBS as a substitute. Prepare serial dilution groups such as 2-fold, 10-fold and 100-fold dilutions. Perform all operations on ice, and mix gently by pipetting; vigorous shaking may lead to protein denaturation.
2.Select a valid dilution factor
Choose the dilution factor that results in OD values falling within the middle linear range of the standard curve for formal testing.

Step 2: Standard Operating Procedures for Retesting (Ensure Valid and Reproducible Data)
1. Retest on the same plate: Load diluted samples, standards and blank controls onto the same microplate. Consistent conditions including incubation, plate washing and color development are recognized by journals and quality control systems.
2.Set replicate wells: Prepare replicate wells for all diluted samples. Calculate the coefficient of variation (CV) to evaluate data consistency. Data with a CV over 10% is deemed invalid and requires retesting.
3. Set dilution control (Advanced operation for high-impact SCI papers): Dilute a qualified sample with known concentration at the same ratio and test it synchronously to calculate dilution recovery and eliminate matrix interference.

Step 3: Data Correction Formula
If the OD value of the diluted sample falls within the linear range of the standard curve:
Original sample concentration = Measured concentration × Dilution factor
Example: An undiluted sample exceeds the quantitative range. After 10-fold dilution and retesting, the measured concentration of the diluted sample is 120 pg/mL.
Original concentration = 120 pg/mL × 10 = 1200 pg/mL

Handling for Samples Below the Lower Limit of Quantitation (LLOQ)
Do not concentrate samples directly, as concentration will enrich the matrix, raise background signals and increase non-specific binding. First check whether samples have undergone repeated freeze-thaw cycles or target analyte degradation. Prepare fresh samples for retesting if sample quality is compromised. For samples with inherently low target analyte levels, adopt the following solutions:

1.Switch to a kit with higher sensitivity.
Samples with concentration below LLOQ were marked as undetectable。
2.For basic research: Uniformly label such samples as < LLOQ in data and figures, and avoid arbitrary value assignment. For methodological validation, clinical tests and high-impact SCI papers: Record results according to the kit’s limit of detection (LOD), or state clearly in manuscripts: Samples with concentration below LLOQ were marked as undetectable.
 

Common Misunderstandings & Corrections
❌ Mistake 1: Analyze and plot data directly using the OD values of diluted samples.
✅ Correction: Always multiply the measured concentration by the dilution factor to restore the original concentration before inter-group comparison.

❌ Mistake 2: Use distilled water or plate washing solution instead of the dedicated sample diluent.
✅ Correction: This will disrupt the antigen-antibody reaction system and cause inaccurate results. Only use the supporting diluent provided by the kit.

❌ Mistake 3: Discard out-of-range samples without any explanation.
✅ Correction: Clearly describe sample dilution procedures in the Methods or Results section of SCI papers.

❌ Mistake 4: Store diluted samples for repeated retests after freeze-thawing.
✅ Correction: Proteins in diluted samples are more susceptible to degradation. Prepare and test diluted samples immediately; do not store them.

Summary
1.Concentration above the linear range: Perform serial dilution → retest on the same plate → verify data with replicate wells → correct concentration by multiplying with the dilution factor.
2.Concentration below the linear range: Use a higher-sensitivity kit, or uniformly label results as below the limit of quantitation/undetectable; do not assign values arbitrarily.
3.General requirements: Use the kit’s dedicated diluent throughout the experiment; adopt stepwise dilution, same-plate testing and replicate wells.
4.For manuscript submission: Specify dilution and data correction methods in detail to reduce the risk of revision.