Laboratory Quality Control: Positive/Negative Controls, Internal Controls and Retesting¶
Key takeaway: Laboratory quality control is the institutional line of defence that makes pigeon molecular test results reliable. Its three pillars are positive and negative controls, internal control monitoring and the retest policy. The positive control verifies that reagents and workflow "go positive when they should", the negative control verifies "stays negative when it should", the internal control monitors whether extraction and amplification are working normally, and retesting catches grey-zone and abnormal results. Without quality control, even the most elegant single Ct value means nothing.
What makes a test report convincing is not just the precision of the instrument but the quality control system behind it. The same Q162D real-time PCR instrument will give results of vastly different credibility depending on whether strict QC is in place. This article breaks down the three pillars of quality control in a pigeon testing laboratory and the practical rules that go with them.
The three pillars of quality control¶
| Pillar | Role | What an abnormal result means |
|---|---|---|
| Positive control | Verifies that reagents and programme work; should be positive | Not positive → failed reagents / wrong programme |
| Negative control | Verifies the absence of contamination; should be negative | False positive → contamination is present |
| Internal control / retesting | Monitors extraction and amplification; catches grey-zone results | Abnormal internal control → extraction or amplification failure |
All three are indispensable in a single run and together form the "triple insurance" behind result reliability.
Positive and negative controls: inside the principle¶
The underlying logic of controls is "verify the unknown with the known": under the same batch and the same conditions, samples of known result are used to prove that the whole testing workflow is performing correctly.
- Positive control: contains a known target nucleic acid (such as a known positive pathogen template or a synthetic target). If it reads positive, the reagents, primers and probes, and the instrument programme are all working properly;
- Negative control: contains no target (such as sterile water or a blank). If it reads negative, no cross-contamination was introduced during the run.
When a control is abnormal, troubleshoot in these directions: a positive control that is not positive → check whether reagents have expired, whether the wrong programme was selected, whether the template was omitted; a negative control that is falsely positive → check for aerosol contamination, shared consumables, and whether zone separation has broken down.
Internal control monitoring: the probe that detects "false negatives"¶
The internal control (internal standard) is the key to monitoring whether extraction and amplification are working. The usual approach is to add a housekeeping gene or a synthetic internal control to the system, so that it is extracted and amplified alongside the target gene.
| Situation | Target gene | Internal control | Interpretation |
|---|---|---|---|
| Normal negative | No signal | Signal present | True negative — the sample really has no target |
| Extraction failure | No signal | No signal | Extraction or amplification failure; result invalid |
| Inhibition | No signal | Weak signal | PCR inhibitors present; the assay needs optimisation |
It is the internal control that lets "no result" be split into "genuinely nothing there" and "nothing was detected" — the crucial bridge between nucleic acid extraction quality and result reliability.
Why false negatives and false positives happen, and how to prevent them¶
Molecular testing has two classic error types — false negative and false positive — each with its own causes and countermeasures:
| Error | Common causes | Prevention |
|---|---|---|
| False negative | Poor sampling, extraction inhibitors, nucleic acid degradation, failed reagents | Standardised sampling, internal control plus positive control, cold-chain storage |
| False positive | Aerosol contamination, sample carry-over, contaminated reagents | Single-direction zone flow, UV disinfection, negative control monitoring |
Aerosol contamination is the number one enemy of a molecular laboratory: if amplification products are handled with the tube open they form an aerosol that drifts and contaminates later reactions. That is why the "closed-tube detection" stressed in qPCR principles is itself a quality control design.
The retest policy and result reliability¶
The retest policy is quality control's safety net. It mainly covers two situations:
- Retesting grey-zone samples: when the Ct value falls in the grey zone (for example 35–40, see Ct value interpretation), re-sample and retest after 3–5 days, or cross-check with a second target;
- Retesting a whole batch when a control fails: if any control is abnormal, the entire batch is void and must be re-run — there is no "selective acceptance".
Laboratories should also keep traceable testing records (sample number, reagent lot number, instrument status, operator) so that results can be reviewed and traced. This is the foundation of race arbitration and of client trust.
In practice: building a laboratory QC checklist¶
- Zoned layout: physical separation and one-way flow across the three zones of sample handling, nucleic acid extraction and amplification/detection;
- Controls in every run: every batch and every plate carries a positive and a negative control;
- Set an internal control: monitor extraction and amplification efficiency and detect inhibitors and extraction failure;
- Always retest the grey zone: retest or cross-check borderline results; never conclude hastily;
- Keep records: retain reagent lot numbers, instrument status and operating records so everything is traceable and reviewable.
💡 Tip: quality control is not "spending more" but "saving the cost of rework and misjudgement". For companion reagents see test kits, for sampling rules see the full sampling and submission workflow, and for testing services see Sanshi Bio testing services.
Key points¶
- Quality control has three pillars — positive/negative controls, internal control monitoring and the retest policy — which together underwrite result reliability.
- The positive control verifies "positive when it should be"; the negative control verifies "negative when it should be"; if a control fails, the whole batch is void.
- The internal control separates "nothing there" from "nothing detected", and is the key to spotting false negatives.
- False negatives usually come from sampling, extraction or reagents; false positives usually come from contamination — each has its own causes and countermeasures.
- Always retest the grey zone and keep traceable records; these are the basis of reliable results and of confidence in competition settings.
Quick reference¶
Molecular testing terms (Chinese–English)¶
| Chinese name | English | Meaning |
|---|---|---|
| 实验室质控 | Laboratory quality control (QC) | The system of rules and operations that keeps test results reliable |
| 阳性对照 | Positive control | A reference containing a known target, verifying reagents and workflow |
| 阴性对照 | Negative control | A reference without target, verifying the absence of contamination |
| 内参 | Internal control / Housekeeping gene | A reference monitoring whether extraction and amplification are normal |
| 复检制度 | Re-test / Repeat testing | The policy of retesting grey-zone and abnormal results |
| 假阴性 | False negative | An error where a truly positive sample reads negative |
| 假阳性 | False positive | An error where a truly negative sample reads positive |
| 气溶胶污染 | Aerosol contamination | Cross-contamination caused by aerosols of amplification product |
| 灰区 | Grey zone / Equivocal | The borderline result range between positive and negative |
| Ct 值 | Ct value (Cycle Threshold) | Cycles needed for fluorescence to reach the set threshold |
| 可追溯性 | Traceability | The property of tracing a result back to sample, reagents and operator |
FAQ¶
What do I do if quality control fails?¶
Locate the cause first. A positive control that does not turn positive points to expired reagents, a wrong programme setting or a missed template addition; a negative control that turns positive points to aerosol contamination, shared consumables or a breakdown of zone separation. Once located, retest the whole batch — never accept only the results you like.
Why do we need both positive and negative controls?¶
The positive control verifies that the assay "goes positive when it should", confirming reagents and workflow are working; the negative control verifies that it "stays negative when it should", confirming there is no contamination. Together they are the batch's quality inspectors — if they are missing or abnormal, the results cannot be trusted.
What does the internal control do?¶
The internal control monitors whether extraction and amplification are working normally. If the target is negative but the internal control is positive, the negative is genuine; if the target and the internal control are both negative, extraction or amplification has failed and the result is invalid. It is the key to spotting false negatives.
What is the grey zone, and how are grey-zone samples handled?¶
The grey zone is the borderline Ct range between positive and negative (for example 35–40). Grey-zone samples should be re-sampled and retested after 3–5 days, or cross-checked with a second target, and never concluded on hastily — see Ct value interpretation.
How does a laboratory prevent aerosol contamination?¶
Strictly enforce physical separation and one-way flow across the three zones of sample handling, extraction and amplification, use closed-tube detection and disposable filtered tips, and carry out regular UV disinfection and decontamination cleaning so amplification products never flow back to contaminate later work.
Can results be used for race arbitration?¶
Molecular test results produced under strict quality control are an important reference for race quarantine and for breeding-stock decisions, but a single result should be judged together with clinical findings and retesting, and the race regulations and relevant standards govern in each case.
Why keep testing records?¶
Traceable records are the foundation of trustworthy results and of client confidence. Sample numbers, reagent lot numbers, instrument status and the operator on duty are all logged, so that problems can be traced and reviewed quickly.
References¶
- Real-time polymerase chain reaction — Wikipedia
- Polymerase chain reaction — Wikipedia
- Analytical quality control — Wikipedia
This content is for general information only and is not a diagnosis or treatment recommendation. Consult your veterinarian or the Sanshi Bio technical team for testing and control decisions (WeChat 15612372623 / email [email protected]).