Standard curves and primer efficiency

A primer pair earns a quantitative claim only after a dilution series has shown how the Cq moves when the input changes. That plot is the standard curve. Its slope is the efficiency. This protocol overview is the decision path for building and reading that curve in a research RT-qPCR assay. It stays at the level of design choices. The mix you pipette still follows the polymerase instructions in your hand, not a volume table copied from an unrelated kit. The place of this assay in a larger study is from cells to a gene expression result, and the relative comparison itself is RT-qPCR for relative expression.

Who needs the curve

You need it when a Cq will be turned into a fold change, a copy-number estimate, or a statement that two assays are efficient enough to share one formula. You also need it when you are choosing among primer pairs. A pair that amplifies something on a gel can still be a poor measuring tool. The curve tells you the measuring range. Reporting that range is part of the transparency described in the MIQE guidelines and, in everyday language, in the MIQE idea in everyday language.

If the biological question is no longer a short gene list, efficiency curves on twenty genes will not substitute for a transcriptome design. That is a different enquiry, framed through the mRNA sequencing enquiry reference and discussed from the quote request.

Slope, doubling, and what efficiency means

Each PCR cycle copies the amplicon. At 100 percent efficiency the copy number doubles, so a tenfold drop in starting molecules delays the threshold by log2 of 10 cycles, about 3.32. On a plot of Cq against the base-10 logarithm of input, that delay is a slope of about minus 3.32. Less efficient copying makes the slope steeper, because you need more cycles to make up for a tenfold dilution. An apparent efficiency above 100 percent, a slope shallower than about minus 3.1, often means the dilution series is not what you think it is: inhibitors in the concentrated standard, a pipetting bias, or a fluorescence artefact.

The conversion used throughout the field is straightforward. If the slope is S, the efficiency as a fraction is 10 to the power of minus 1 over S, minus 1. Multiply by 100 for a percentage. You do not need a special instrument to do that arithmetic. You do need the points to be a line across the dilutions you care about.

A window of 90 to 110 percent is widely used when people plan an assay. Treat it as a planning window. The assay's own validation decides whether a value inside that window, or a stated distance outside it, can support your comparison. Two assays at 90 and 110 percent are both inside a common window and are not interchangeable in the simplest doubling formula. That consequence is the subject of delta-delta Cq and what it assumes.

What you dilute, and the equipment class

The standard should resemble the samples. A pooled cDNA from the same reverse transcription is often the fairest template for a relative expression assay, because it carries the same priming history. A plasmid or a synthetic oligonucleotide can define a copy-based curve, and it can look more perfect than the biological matrix. If you use it, remember that it may lack the inhibitors and the competing nucleic acids of a real extract. Say which template class you used.

Dilute in the buffer the protocol recommends for that mix, across several steps, commonly covering a few orders of magnitude that bracket the unknown samples. Exact volumes belong to the pipette check and the manufacturer's protocol. The decision is the spacing: each step should be large enough that Cq is expected to move, and small enough that you can see where the line bends. Run replicates at each point so a single mis-pipette cannot define the slope.

The instrument is a real-time cycler with an optical block matched to the plate or tubes. Thin-wall vessels and an optical seal are part of the measurement, because the camera is reading through them. A master mix class supplies polymerase, buffer, magnesium environment and a detection chemistry, usually an intercalating dye or a hydrolysis probe. Dye chemistry needs a melt or a gel to show that one product is responsible for the signal. Probe chemistry is more specific and can still be inefficient if the amplicon or the probe binding is poor.

Primers are the specificity. Design them against a named accession and inspect other binding sites with NCBI Primer-BLAST. A curve on the wrong transcript is a smooth line about the wrong thing.

Branches when the control fails

Plot the points before you average away the trouble. If the concentrated end of the series flattens, suspect inhibition or too much template. Drop those points from the reported range, or purify and rebuild. If the dilute end scatters or merges with the no-template control, you have left the dynamic range. The dynamic range is the interval where the line holds, not the interval you hoped to cover.

The no-template control contains every reagent except template. It should not cross the threshold. If it does, separate contamination from primer dimer. Dimers often melt at a lower temperature than the amplicon and show up as a small product. They dominate when real template is scarce, so they bend the bottom of the curve and invent signal in negative samples. A failed no-template well stops the plate. Clean the workflow, adjust annealing within the enzyme's guidance, or replace the primers. Do not subtract the dimer and call the curve valid.

R-squared, or an equivalent fit statistic, describes closeness to the line. Laboratories like a tight fit. A tight fit with a slope that implies 70 percent efficiency is a repeatable poor assay. Redesign the primers or shorten a difficult amplicon rather than memorising a bad slope. A loose fit means pipetting, sealing, or a mix that is not homogeneous. Repeat the series before you publish the efficiency.

Unknown samples that fall outside the validated range are not quietly extrapolated. Dilute them into the line, or accept that the assay did not measure them. A sample that amplifies later than the no-template control is not a low biological result.

A polite curve that is still the wrong assay

Efficiency can drift between a plasmid standard and a cDNA full of biological background. If you will quantify cDNA, show at least that a cDNA dilution behaves. Efficiency can also differ between a purified calibrator and a crude lysate. Inhibitors often hide in the least diluted point and make the top of the curve too flat, which pushes the apparent efficiency upward. Removing that point changes the slope. Report the points you kept.

A shared threshold and baseline across the curve and the unknown plates matter. Calling Cq with different software rules on the standard and the samples invents a second efficiency. Freeze the analysis settings in the notebook beside the slope.

Primer dimers deserve a second look when the biological samples are as dilute as the bottom standard. A curve built on abundant template can hide a dimer that appears as soon as the real target drops. Always look at the no-template melt on the same run as the precious low-expression samples.

Public workflow notes on protocols.io show how other laboratories lay out a dilution series. Copy the logic of controls, and follow the cycling window your own enzyme specifies.

Research limits and the room the cycler sits in

A validated curve supports a research measurement inside its range. It does not turn the assay into a diagnostic test, and it does not set a biosafety level for the template. Infectious or human material stays under institutional rules. Hot blocks, optical seals and any intercalating dye are handled under the hazard notes that come with them. Ultraviolet inspection of a gel, if you use one to confirm the product, needs the shielding the transilluminator requires.

Humidity on an optical seal

In a humid room, a plate that waits for its seal can collect condensation, and a poorly seated film leaves wells that evaporate. Both change fluorescence. Seal promptly, spin the plate as your instrument guidance suggests, and rerun a known standard if a cycler rebooted after a power cut mid-protocol. Two blocks set to the same programme can still disagree if calibration and plastic differ. The efficiency you trust is the one measured on the instrument you will use for the samples.

What to send when you still need reagents or a larger study

An enquiry for mixes, probes or plates should name dye versus probe chemistry, the instrument family, the organism, and whether you are still at the curve-building stage. Start from the molecular biology catalogue and the nucleic acid analysis pathway. If the curve work is only a confirmation after a transcriptome contrast, name that contrast as well.

The differential expression analysis enquiry reference and the mRNA sequencing enquiry reference are enquiry references when sequencing is the actual question. Put the assay status, the species and the comparison on the quote request. A method can be discussed there. The request is how you ask whether a quotation is possible.