Procedure using control samples means running a known reference material alongside your test samples to verify that your method, equipment, and reagents are working correctly. This is the core of quality assurance in any analytical lab, from clinical diagnostics to environmental testing.
What Are Control Samples?
Control samples are materials with known properties (e.g., concentration, identity, or physical state) that you process exactly like your unknown samples. They are not calibrators—calibrators are used to build a standard curve, while controls are independent checks that the curve is valid. Controls can be purchased from commercial suppliers, prepared in-house, or obtained from certified reference material providers.
There are three main types:
- Positive controls: contain the target analyte at a known level and should give a positive result.
- Negative controls: lack the target analyte and should give a negative result.
- Matrix controls: mimic the sample matrix (e.g., blood, soil, water) to check for matrix effects.
Why Use Control Samples?
Controls catch errors before they affect your results. They detect problems like reagent degradation, instrument drift, operator mistakes, or contamination. In regulated industries (clinical labs, food safety, environmental monitoring), controls are mandatory for accreditation and legal defensibility. Without them, you cannot trust any result—even if it looks plausible.
Step-by-Step Procedure
The exact procedure varies by field, but the general workflow is:
- Select appropriate controls—choose a positive, negative, and matrix control that bracket your expected range.
- Prepare controls—follow the manufacturer’s instructions or your validated in-house protocol. For liquid controls, mix gently; for lyophilized controls, reconstitute with the specified diluent.
- Place controls in the run—include at least one control at the beginning and one at the end of each batch. For large batches, place controls every 20–50 samples.
- Process controls exactly like samples—same reagents, same volumes, same incubation times, same instrument settings.
- Read control results—compare them to the established acceptable range (mean ± 2 or 3 SD, or manufacturer’s target).
- Evaluate the run—if controls pass, accept the sample results. If they fail, reject the run, troubleshoot, and repeat the entire batch.
How to Choose the Right Control Samples
Choosing controls is a matter of matching your needs:
- Matrix: use controls with the same matrix as your samples (e.g., serum for blood tests, water for environmental samples).
- Concentration: pick controls near the decision point (e.g., cutoff for a positive/negative test) and at least one in the normal range.
- Stability: check expiry dates and storage requirements. Expired controls are a common cause of false failures.
- Traceability: for regulated work, use controls with documented traceability to a reference method or certified reference material.
Common Problems and Troubleshooting
Even with controls, things go wrong. Here are typical issues:
- Controls out of range: check reagent lot, calibrator drift, pipetting accuracy, or instrument maintenance. Run a new control vial to rule out degradation.
- Negative control positive: indicates contamination—check for cross-contamination between samples, dirty glassware, or contaminated reagents.
- Positive control negative: suggests reagent failure, wrong dilution, or instrument malfunction.
- Trends: if controls are drifting toward the limit but still in range, investigate before they fail—this is the basis of Levey-Jennings charts.
Control Samples vs. Standards vs. Blanks
People often confuse these terms. Here is the difference:
- Standard (calibrator): used to create the calibration curve—its value is assigned by the manufacturer.
- Control: used to verify the calibration—its value is known but not used for calculations.
- Blank: contains no analyte and is used to detect background interference or contamination.
In practice, you run all three in a batch. The blank tells you the baseline, the standards define the curve, and controls confirm the curve is still valid.
Cost and Safety Considerations
Controls add cost—typically 5–15% of reagent budget—but they save money by preventing rework and false results. Safety is also a factor: control materials may be biohazardous (e.g., human serum) or toxic (e.g., pesticide standards). Always follow your lab’s safety data sheets and dispose of controls as hazardous waste if required.
Frequently Asked Questions
How often should I run control samples?
At minimum, run controls at the start and end of each batch. For longer runs, add controls every 20–50 samples. In clinical labs, controls are often run every 8 hours or with each shift change. If you change reagent lots, run new controls before proceeding.
What if my control fails?
Stop the run and do not report any sample results. Investigate the cause—check the control vial, reagents, pipettes, and instrument. Once you identify and fix the issue, rerun the entire batch with fresh controls. Never just subtract the control failure and report the rest.
Can I make my own control samples?
Yes, but only if you validate them. In-house controls must be tested for homogeneity, stability, and target values. They should be aliquoted and stored properly. For regulated work, commercial controls are often preferred because they have documented traceability.
What is a Levey-Jennings chart?
It is a graph of control results over time, with lines at the mean and ±1, 2, and 3 standard deviations. It helps you spot trends (e.g., gradual drift) and shifts (sudden change) before controls actually fail. Many labs use software to generate these charts automatically.
Template for Control Sample Log
| Date | Control ID | Lot # | Expiry | Expected Value | Observed Value | Pass/Fail | Technician |
|---|---|---|---|---|---|---|---|
Sample (fictional): 2024-05-01 | C-123 | L456 | 2025-01-15 | 25.0 µg/L | 24.8 µg/L | Pass | J. Doe
Always record control results immediately and review them before releasing sample data. If you are in a regulated field, retain these logs for audits.