Troubleshooting a PCR with no band

A PCR with no band is a gel that shows marker fragments, or at least shows that the lane ran, and shows no product where the amplicon should be. The useful next step is to name the failure with a word that predicts a different test. Dropout, inhibition, mis-priming, dead enzyme and missed activation are those words. This glossary defines them inside one investigation, not as a list to memorise. The reaction you are judging is the endpoint cycle in how polymerase chain reaction works. A blank lane is not yet a biological negative.

Start with the gel, then the positive control

Confirm you are looking at a PCR failure and not a gel failure. If the marker is invisible and the tracking dye did not move, the power supply, the buffer or the loading is the problem. No sample call is available. If the marker is sharp and every PCR lane is empty, the cycler reactions are the problem.

Next split the empty lanes with a positive control that uses the same master mix and a template you trust. Controls of this kind are set out in PCR controls and contamination control. The branch is simple. When the positive control is also blank, the samples have not been tested. Look at dead enzyme and missed activation before you invent a story about every specimen. When the positive control shows the expected size and the samples do not, the mix can work. Look at inhibition, true absence, and dropout.

The no-template control should be blank at the product size in either branch. If it is not, you have a contamination or primer-artefact problem riding alongside the blank samples, and you fix that before you interpret a later repeat.

Dropout

Dropout means the target class was supposed to be amplifiable and this tube produced no detectable product. In a multiplex it often names one missing band while the others appear. Allele dropout is narrower: a mismatch under a 3-prime end lets one allele extend and leaves the other invisible. The lane can look like a clean homozygote, or like a total failure if you had only the mismatched allele.

The test is the primer sequence against the alleles you might have, not another ten cycles. If the 3-prime end covers only one variant, redesign that end or accept the blind spot. A blank called negative without that check turns a polymorphism into a false absence.

Inhibition

Inhibition means a component of the tube reduced polymerase activity or primer availability. Heme, humic acids, ethanol or salt carried from an extraction, EDTA, too much DNA, and residual detergent are typical members of the class. The enzyme is alive. The positive control, which did not receive the nasty matrix, works. The sample lane is blank or weak.

Dilute the sample, and spike a known positive template into the extract. If the spike dies, the extract is inhibitory and a blank lane does not mean the target is missing. If the spike survives and the sample stays blank, absence is a fairer word, still limited to this assay. Endpoint PCR does not report a copy number either way.

Mis-priming

Mis-priming is extension from a primer bound at an unintended site. The product is the wrong length, or one of several lengths. You diagnose it when extra bands appear, especially at a low annealing temperature, and when a higher temperature or a new 3-prime end removes them. It is the wrong label for a lane that contains nothing.

A completely empty lane at a high annealing temperature is a window you missed. The primers never stayed bound. The next experiment is a lower set-point or a better 3-prime match. Mis-priming would have left DNA you did not want. A blank leaves none. The remedies point in opposite directions.

Dead enzyme

Dead enzyme means the polymerase no longer extends, because it was left warm, freeze-thawed past usefulness, or never added. Nucleotides and primers can be "dead" in the same practical sense: omitted, degraded, or the wrong stock. The signature is a blank positive control and blank samples in a mix that should have worked. A parallel reaction with a fresh enzyme aliquot, same primers and same positive template, is the split. If the fresh aliquot restores the band, retire the old tube. If it does not, the shared primers, dNTPs or programme are next, one change at a time.

Mark the failed tube with the date and take it out of the shared box. A later user will otherwise rediscover it as a new mystery.

Missed activation

Missed activation is specific to hot-start enzymes. An antibody, a chemical modification or an aptamer holds the polymerase until a heat step releases it. If that hold never ran, or ran for too short a time for a chemical block, the enzyme stays shut and every lane is blank, including the positive control. It looks identical to a dead enzyme until you read the programme.

Open the cycler history. Confirm the initial temperature and the minutes the card asked for. A reboot can drop the first step and start at cycle one. Chemical hot-start cards are the least forgiving. Antibody releases often coincide with a normal initial denaturation, so a very short denaturation can still be too short. If the history shows the hold completed and a second enzyme lot also fails, drop this hypothesis and return to a dead mix or a bad primer. If the history shows the hold was skipped, repeat the programme before you reorder anything.

Words that do not belong on a blank lane

Absence is what you may say after inhibition and dropout have been tested, and only as not detected by this assay. Contamination explains a band, not emptiness. Over-cycling does not erase a product that was never made. If the positive control works and the sample shows neither band nor smear, replace the template rather than renaming the failure.

Safety and a freezer that warmed

No-band troubleshooting uses the same hot block and the same stained gel as a successful run. Ultraviolet viewing still needs the shield. An infectious template is an institutional containment decision. The WHO biosafety topic is background, not permission to work with a pathogen, and a blank PCR is not a clinical negative.

Power cuts complicate the enzyme word. A freezer that warmed can leave an aliquot looking frozen after it has already thawed. That tube is a candidate dead enzyme, not a mystery of the primers. Mark thawed lots and do not pour them back into the unopened stock. If the cycler rebooted, read the activation step in the run log before you call the enzyme dead. Those two checks are cheap because they stop a glossary from becoming a shopping list.

What to tell a supplier

Say which control was blank, the enzyme class, whether a hot-start hold is required, the amplicon length, and whether a spike into the sample died. The molecular biology catalogue is where enzyme families are grouped. Put the control result on the quote request so the answer is about the failed branch, not about PCR in general. A difficult matrix belongs with the nucleic acid analysis pathway. If one target in a multi-target mix is the only blank, say that in the multiplex PCR enquiry reference. The missing band may be competition, which is a different word from a dead enzyme.