Inhibition and weird Cq values

The Cq was 34 in a sample you expected to be abundant, and a neighbouring well of the same RNA was 26. Another well reported a Cq of 8, which is earlier than the assay has any business being. Both numbers are weird until you know which check they survive. This selection guide matches the oddity to a test: dilute, spike a known template, or read the melt. It is not a recipe of volumes. How a relative assay should be built when the numbers are boring is in RT-qPCR for relative expression. The wider study is in from cells to a gene expression result.

Pick the check that matches the odd number

A quantification cycle is a crossing point, not a concentration. Weird means the crossing disagrees with a control you trust, or the shape disagrees with the number. Selecting a check is the whole skill. Diluting everything by habit wastes the plate. Spiking everything wastes template. Looking only at the number, and never at the melt or the raw curve, is how a primer-dimer becomes a fold change.

Three checks cover most research plates.

Dilution of the sample. If an inhibitor is the cause, diluting the well often improves amplification by more than the dilution removes template. The Cq comes back earlier than the dilution factor predicts. If the sample is simply scarce, dilution makes the Cq later, in line with the factor, for as long as you remain inside a clean efficiency.

A spike of known template. Add a target you can recognise, or use an internal control already in the mix, and compare the spike in the sample matrix with the spike in clean buffer. A delay only in the matrix is inhibition or loss. A spike that is fine in the matrix means the late Cq of your gene is more consistent with low template.

A melt or a size check. Dye-based chemistry fluoresces for primer-dimer and for the real amplicon. An early or late Cq with a low-temperature melt is not abundance. Probe chemistry ignores some dimers and can still be fooled by the wrong target that binds the probe. Match the check to the chemistry you ran.

The MIQE guidelines expect inhibition and specificity to be addressed in the record. They do not pick the check for you.

What an inhibitor does in the well

Polymerase and reverse transcriptase slow down when heme, excess salt, phenol, or ethanol from a cleanup is still in the tube. The curve may rise late, rise with a shallow slope, or stall. A shape that is steep and then flat can be an ordinary plateau after a healthy exponential phase. Plateau itself is not a defect. The suspicious cousin is a curve that never shows a clean exponential phase, or that plateaus at a fluorescence far below the rest of the plate. Partial inhibition, a reagent that ran out, and an optical problem can share that drawing. The raw view separates an optical problem from a chemical one.

An internal control that fails is more informative than a target that is late. If the control was spiked at the qPCR stage and it is late only in some wells, those wells are inhibited or mis-piped. If the control was spiked into the sample before extraction and it is low, you cannot yet separate extraction loss from inhibition. That is a different branch: the molecule never arrived, or it arrived and could not amplify. A second spike at the qPCR stage splits them.

Reference genes do not rescue this. A housekeeping transcript that is late because the well is inhibited will scale the target into a confident, wrong fold change if you treat it as biology. If target and reference move together in a way that tracks the spike, believe the well, not the gene.

Dilution, spike and melt as classes of check

You are choosing among classes, not assembling a vendor table. A dilution series uses the same mix and several folds of the sample. Follow the mix instructions for how much template is even allowed. Too much RNA inhibits on its own. The arithmetic to keep in your head is simple: each doubling of dilution is about one cycle later when efficiency is near one doubling per cycle. A four-fold dilution is about two cycles. A ten-fold dilution is a little more than three. If the observed Cq is substantially earlier than that prediction, the concentrated well was suppressed. If your efficiency is known to be poor, the expected shift changes. Do not quote a textbook shift as a pass-fail law for a tired primer pair.

A spike uses a template that should be constant. The cleanest comparison is spike-in-buffer versus spike-in-sample, same input of the spike. The gene of interest can stay in the well or be discussed separately. What you are selecting is evidence about the matrix.

A melt uses the dye plate you already ran, if the instrument collected a dissociation. The specific amplicon has a characteristic temperature. A primer-dimer melts lower. Two peaks mean two products, and one Cq cannot represent both. If you did not collect a melt, an endpoint gel is the same class of check. Skipping both is how early cycles get misread.

Branch on the shape and on which wells failed

All samples late, including a clean positive control. This is not sample inhibition. The mix, the channel, the primers or the cycler is the problem. Flat-curve logic in troubleshooting flat amplification curves is the neighbour of this branch.

One sample late, its spike also late, other samples fine. Select dilution and a cleanup. Do not compare that Cq with a clean calibrator and call the gene down. Heme, salt and ethanol are the usual guests. Blood-derived RNA is a frequent heme source. A column elution that still smells of ethanol was not dried. Excess salt can come from a precipitation that was not washed.

One sample late, its spike on time. Select "low template" as the working idea, and still look at the melt so the late cycle is the right product. A late dimer can imitate scarcity.

Cq absurdly early, melt is a low peak. Select primer-dimer or off-target. Do not report the gene as wildly induced. Redesign or raise specificity under the mix instructions. Contamination with a concentrated standard also produces early cycles, and the melt will match the real amplicon. The no-template well is the branch: if it is early too, the plate is contaminated, which is the territory of PCR controls and contamination control.

Steep rise then a normal plateau, Cq in the range you expect, melt correct. Select "this one is fine". Do not chase inhibition because the curve has a flat top. Every successful qPCR has a flat top.

Symptom to check

Early numbers that are not abundant template

The weird early Cq is the one people trust too fast, because it looks like a strong result. If you only watch the number, a primer-dimer that amplified efficiently is indistinguishable from a loaded sample. Open the melt. If the peak is not the amplicon you characterised on a clean template, discard the Cq. A probe assay may not show that dimer. It can still report an early cycle from a contaminated standard. The no-template well is mandatory either way.

A reference gene with an early dimer and a target with a real amplicon will invent a huge fold change. The maths is obedient. The biology is not there.

Research traces only

Inhibition troubleshooting does not create a diagnostic qPCR, and it does not authorise work with blood or other human material outside the containment your institution sets. The laboratory biosafety manual is a public framework institutions use when they write those rules. It is not a permit. Heme is a biochemical inhibitor and blood is a biosafety question. Keep those decisions separate. Follow the safety notes for the extraction chemicals that often accompany the inhibitors: chaotropes, alcohols and stains.

Heme, salt, ethanol and slow drying

Ethanol carryover is more stubborn when the air is humid and a "short spin to dry the column" was timed in a dry climate. If the elution smells of alcohol, or the Cq pattern matches inhibition only in the samples that were extracted that day, dry longer or change the cleanup, under the extraction instructions, and rerun the spike. Do not bake a column at an invented temperature. Heat that is not in the insert damages RNA, and then you have two problems. Salt behaves similarly: an extra wash is a method choice from the kit, not a folklore rinse.

Blood and haemolysed tissue deserve the spike by default, not after a week of strange fold changes. Put the known template into the matrix early. Addgene's protocols show how often other laboratories write that control in. Copy the habit, not somebody else's volumes.

A power cut during a qPCR run produces partial curves and nonsense crossings. Those are not inhibited samples. Discard the crossings and rerun when the block is trustworthy.

What to include when a plate looks inhibited

Say which wells were late or early, whether the spike matched them, what the melt showed, and what the sample matrix was. Mention blood, a heavy salt prep, or a rushed ethanol dry if those are true. A targeted-assay discussion can sit beside the mRNA sequencing enquiry reference if you are deciding that sequencing is the wrong next step until the RNA is clean, and beside the differential expression analysis enquiry reference only when the comparison itself is the deliverable. The pages are prompts for method talk.

Mixes and oligos are classes in the molecular biology catalogue. Matrix and assay together belong to the nucleic acid analysis pathway. Describe the weird Cq and the check you already ran in the quote request.