Positive negative and no-template controls

Positive, negative and no-template controls answer three different questions. This page is the application decision: which of the three you run, the order in which you build them, and the moment you stop reading sample lanes. How a shared bench gets those controls dirty is the companion note on PCR controls and contamination control. The cycle they sit inside is how polymerase chain reaction works.

Use the molecular biology catalogue when the missing piece is an enzyme, a nucleotide mix or a vessel, and send the control plan with the quote request.

When each control earns a tube

Run a no-template control on every setup. It contains the master mix, the primers and the water or buffer you use in place of sample. It asks whether those reagents, handled the way you handled them today, can produce the assay product with no template added. If you skip it, a band in a sample has no contrast. You will not know whether the band was already in the mix.

Run a positive control whenever a negative sample would mean something. It contains a trusted template the primers should amplify: a plasmid of known sequence, genomic DNA from a known source, or a previously checked fragment used sparingly and stored away from the mix bench. If the positive fails, the enzyme, the programme, the primers or the seal is the suspect. Calling the samples negative in that run is the wrong sentence.

Run a negative control when the sample path itself can create a band. This is the tube people collapse into the no-template control, and the collapse is the mistake. A negative control is a matrix you expect to lack the target. An extraction blank, carried through lysis and elution beside the specimens, is the usual form. A known non-target DNA is another, when the question is cross-reaction. It asks whether collection tubes, buffers or a shared pipette created product. The no-template control never sees those steps, so it cannot answer for them.

You can stop adding controls only in a narrow sense. A qualitative presence assay on a clean plasmid, with a fresh mix, still keeps the no-template tube. What you may drop, once a preparation method has a history on that assay, is a spike on every single sample. You may not drop the no-template control because the last month was clean.

Place them before the samples, and add the positive last

Write the rack map before any tube is open. The no-template position, the negative position, the sample positions and the positive position are fixed on paper first. That is what "controls before samples" means at the bench: the decision is already made, so a hurried extra sample cannot steal the blank's well.

Pipette the no-template control and close it before template tubes are open. Then set the negative control and the samples. Add the positive template last, and close it at once. The positive is a concentrated source. Adding it first is how a clean plan contaminates the blank it was meant to protect. Gloves that have touched the positive do not return to the blank.

After the run, read the no-template control first. If it shows a product at the amplicon size, stop. Sample lanes are not interpretable as positives. If it shows only a short, fuzzy product, name that as primer-dimer and do not confuse it with the assay band. Primer-dimer can appear in a blank because the primers primed each other. A full-length band in a blank is carryover or a contamination of the mix until you have a better explanation.

Then read the negative control. A band there means the sample path is contaminated even if the no-template tube is clean. Then read the positive. A miss there blocks every negative call. Only then read samples, and only as presence and size. Endpoint brightness is not a count of starting copies.

A clean no-template control does not prove every sample tube was clean. It speaks for the shared mix. A tip that touched only one sample, a glove that brushed one rim, or a splash while the blank was already closed can still seed that one tube. If a single sample looks wildly unlike its replicates, suspect that tube before you invent a biological story.

Branch points

If the no-template control is dirty, retire the open aliquots of mix, water and primers. Do not repeat the samples with those aliquots to "confirm" the bands. How to break the contamination path is covered in the companion controls note. Your job at this decision point is to stop the call.

If the no-template control is clean and the positive fails, change one named thing: enzyme aliquot, primer dilution, or a programme you have compared with the enzyme card. Annealing temperature depends on primers, salt, magnesium and additives. A calculated melting temperature is a start, not a guarantee that this mix matches the paper.

If the positive works, the blanks are clean, and a sample is negative, ask whether the matrix can inhibit. A spike of a known target into an aliquot of that sample, or a dilution, separates a dead tube from an absent target. Until you have done one of those, "negative" is provisional. The application stops short of a clinical negative in every case.

What the three tubes cannot do

They cannot prove sequence identity. A sample band at the expected size, beside a clean blank and a working positive, is consistent with the design. A similar-sized side product still fits those observations. Sequence the product, or cut it with a diagnostic enzyme, when identity is the claim.

They cannot rescue a primer pair that dimerises strongly. Hot-start, whether antibody, chemical or aptamer, reduces extension while you pipette. It does not redesign the oligos. A dimer in the no-template lane is information about the primers. Keep it.

They cannot assign a clinical or forensic meaning. A complete control set makes a research result interpretable inside the question you wrote. Diagnostic use needs a validated assay and the legal framework that applies where you work.

Safety

Template from an infectious specimen is not made safe by being a "control". The positive control is often the highest copy-number tube on the rack and the one most able to seed the next run. Store it away from the master-mix area, open it last, and follow institutional biosafety rules. The WHO laboratory biosafety manual is background, not a permit.

What to send with an enquiry

Name the three controls you intend to run, the template the positive will be, whether the negative is an extraction blank, the amplicon length, and whether the assay is endpoint or real-time. Enzyme class and hot-start belong in the same note. The nucleic acid analysis pathway covers the path from sample to gel or trace. If several targets share a tube, use the multiplex PCR enquiry reference as a question prompt. It is not a statement that the assay is already running. Ask whether a quotation is possible.