Skip to main content

Table 1 Summary of QC sample types, interpretation, and possible actions

From: Wrangling environmental exposure data: guidance for getting the best information from your laboratory measurements

QA/QC ConceptMeasureInterpretationPossible Actions
AccuracyLab control sample recoveries and/or matrix spike recoveries
Certified reference material
Isotope dilution quantification
Measure of whether the analytical method produces accurate quantification for each compound. Matrix spike recovery evaluates matrix effects on accuracy, such as interferences.
Isotope dilution is the most rigorous approach to generating accurate measurements in biomonitoring.
• Drop compounds with inaccurate quantification from the data analysis, discuss with lab whether improvements can be made for future analyses.
• If problems are modest and batch-specific, include batch as a covariate in regression model.
Extraction efficiencySurrogate spike recovery in each sampleMeasure – for each field sample - of whether the chemical is extracted completely from the sample matrix, (e.g., blood, dust).
Isotope dilution approaches capture and correct for differences in extraction efficiency.
• Consider dropping samples with poor surrogate recovery from data analysis.
• Consider applying a surrogate correction factor (1/fraction recovery) if the recovery is consistent (± 15–20% in standard deviation).
Detection limit Level above which the lab can detect with confidence that the analyte is present in the sample.
Common terms include:
Instrument detection limit (IDL), Detection limit (DL), Method detection limit (MDL), Limit of detection (LOD)
• See Method Reporting Limit.
Quantitation limit Level above which the lab can quantify with confidence the amount of chemical in the sample.
Common terms include:
Practical quantitation limit (PQL), Limit of quantitation (LOQ), Laboratory quantitation level (LQL), Contract required quantitation limit (CRQL)
• See Method Reporting Limit.
Method Reporting Limit (MRL)Levels detected in blanks (lab-blind field blanks, solvent blanks, matrix blanks, storage blanks, other types)Level above which the researcher is confident that the reported chemical measurement reflects a signal from the media sampled, considering all sources of measurement error, especially potential contamination during sample collection and handling as well as in the laboratory.• Determine MRL by comparing the lab limit (quantitation limit, unless not reported, in which case detection limit) to the levels in the blanks for each compound.
• Qualify reported values below the MRL as “estimated”.
Potential contamination / Analytical biasLevels detected in blanks (lab-blind field blanks, solvent blanks, matrix blanks, storage blanks, other types)Measure of confidence in accuracy of values reported above the MRL.• If evidence of contamination, consider dropping a compound or dropping results for a compound in a particular batch.
• Identify source of contamination (e.g., lab vs field equipment) to inform future work.
• For compounds with consistent contamination in blanks, researchers may correct field sample quantity by subtracting the amount attributed to contamination. This is most important when contamination is significant relative to sample values (e.g., > 10%) and for comparisons with external data.
PrecisionRelative percent difference (RPD) for side-by-side duplicate samples (lab-blind) or split samples (lab-blind if possible)A measure of reproducibility of field measurements, including analytical variability and sampling variability.• Flag compounds with > 30% RPD.
• Consider precision in combination with other QA/QC when deciding to qualify results.