Dot blot protocol: how to run a dot blot and read the results
A dot blot detects a protein by spotting the sample straight onto a membrane and probing it with an antibody, without running a gel first. To run one, spot 1 to 2 microliters of each sample and a dilution series of a reference sample onto nitrocellulose or PVDF, let the spots dry, block, incubate with the primary and then the secondary antibody, and detect. To read it, compare each spot with a blank and with your standards, and keep every spot you quantify inside the linear range.
This guide gives the protocol step by step with amounts and times, then shows how to read the spots, how to turn them into numbers, how a dot blot differs from a western blot, and how to fix the problems that come up most often.
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The dot blot protocol at a glance
| Step | What to do | Typical amount or time | Why |
|---|---|---|---|
| 1. Membrane | Nitrocellulose, or PVDF wetted in methanol | Cut to fit the grid | Nitrocellulose gives lower background; PVDF can be reprobed |
| 2. Grid and layout | Mark a light pencil grid and record what goes where | One spot per sample per replicate | You need to know which spot is which when you read and quantify |
| 3. Samples and controls | Equal protein per sample, plus a dilution series of a pooled reference, a positive control, a negative control and a buffer blank | 0.25 to 2 µg lysate per spot to start; duplicates or triplicates | Controls show background, specificity and the linear range |
| 4. Spot and dry | Apply slowly, tip just above the membrane, then dry completely | 1 to 2 µL per spot; 15 min at 37 °C or 45 min at room temperature | Small round spots are easier to read and measure |
| 5. Block | 5% non-fat milk in TBST (BSA for phospho-specific antibodies) | About 1 hour | Stops antibodies binding the bare membrane |
| 6. Primary antibody | Diluted in blocking buffer or TBST | Overnight at 4 °C or about 2 hours at room temperature | Binds the target protein |
| 7. Wash | TBST | 3 washes | Removes unbound antibody and lowers background |
| 8. Secondary antibody | HRP- or fluorescently labeled, matched to the primary's host species | 1 to 2 hours, then 3 more washes | Carries the signal you detect |
| 9. Detect and image | ECL substrate or fluorescence imaging | Longest exposure with no measured spot saturated | Saturated spots cannot be measured; keep the raw image |
A protein dot blot protocol, step by step, with typical amounts, times and the reason for each step. Optimize the antibody dilutions and the load for your own target.
What a dot blot is, and what it is for
A dot blot, or dot immunobinding assay, puts a small drop of sample directly onto a membrane, where the proteins bind. The membrane is then blocked and probed with antibodies exactly as in a western blot. Richard Hawkes and colleagues described the method for screening antibodies in 1982 (ref 1). The nucleic acid version, dot hybridization, came a few years earlier: a labeled probe is hybridized to DNA or RNA spotted on the membrane, which is how DNA and RNA dot blots still work (ref 2).
Because there is no gel, a dot blot is quick, needs very little sample, and fits dozens of samples on one membrane. In one study, about 1 microgram of lysate per spot gave a usable signal, against about 40 micrograms per lane for the western blot of the same samples (ref 3). Labs use dot blots to:
- check whether a protein is present in many samples at once,
- screen and titrate antibodies before western blotting,
- compare the amount of a protein across many samples, once the antibody is known to be specific,
- read cytokine and antibody arrays, which are dot blots with a fixed layout.
The trade-off is that nothing is separated. Everything in the sample that the antibody binds ends up in the same spot.
A DNA microarray takes the same idea to thousands of spots of known DNA sequences on a glass slide or chip: see how a DNA microarray works and how its images are analyzed.
Dot blot vs western blot
| Feature | Dot blot | Western blot |
|---|---|---|
| Gel separation | None: sample spotted directly | Proteins separated by size on a gel first |
| Shows protein size | No | Yes, against a molecular weight ladder |
| Reveals off-target binding | No, it adds to the same spot | Yes: extra bands appear at other sizes |
| Sample per measurement | About 1 µg of lysate in 1 to 2 µL | Typically 20 to 50 µg per lane |
| Samples per membrane | Dozens to hundreds | Typically 10 to 15 lanes on a mini gel |
| Time | One day, or two with an overnight primary | Usually two days, including the gel and transfer |
| Quantification | Relative, or absolute with a protein standard, once the antibody is validated | Relative to a loading control |
| Best for | Screening and titrating antibodies, many samples, arrays | Confirming identity and size, specificity, complex samples |
How a dot blot and a western blot differ, and what each is best used for.
The difference that matters most is specificity. On a western blot, a band at the wrong size tells you the antibody has bound something else. On a dot blot that signal is added to your spot and you cannot see it. So before you rely on a dot blot to compare samples, confirm on a western blot that the antibody gives one band at the expected size in the same kind of sample (ref 3). Once that is done, a dot blot can measure the same protein in far more samples, with far less material, than a western blot. In the same study, dot blot and western blot results for the same samples agreed closely (ref 3).
If your work needs the band size, or the antibody gives more than one band, run a western blot instead. How to read a western blot covers what each band is telling you.
Dot blot protocol, step by step
- Choose the membrane. Nitrocellulose binds protein strongly and gives a low background, but it is brittle and is not suited to stripping and reprobing. PVDF is tougher and can be reprobed, but it gives a higher background and needs thorough washing (ref 5). Wet PVDF in methanol first; nitrocellulose goes in dry.
- Mark the grid. Draw a light pencil grid on the membrane so you know where every spot belongs, and record the layout before you start. A fixed layout also makes quantification much faster later.
- Prepare samples and controls. Measure total protein so every sample is loaded at the same amount. Alongside your samples, spot:
- a dilution series of a pooled reference sample (or a purified standard), to find the linear range and to calibrate,
- a positive control that contains the target,
- a negative control that lacks it, such as a knockout lysate,
- a buffer-only blank, to show the background.
Run each sample in duplicate or triplicate. For a single target, 0.25 to 2 micrograms of lysate per spot is a typical starting range, but the linear range depends on the antibody (ref 3).
- Spot the samples. Apply 1 to 2 microliters per spot (ref 3), slowly, with the pipette tip just above the membrane, so each spot stays small and round. For larger volumes, apply them a microliter at a time and let each drop soak in, or use a vacuum dot blot manifold. Let the membrane dry completely, about 15 minutes at 37 °C or 45 minutes at room temperature (ref 3).
- Block. Rinse in TBST, then block for about an hour in 5% non-fat milk in TBST (refs 3, 5). Use BSA instead of milk for phospho-specific antibodies, because milk contains the phosphoprotein casein (ref 5).
- Primary antibody. Incubate overnight at 4 °C or for about 2 hours at room temperature, with gentle agitation (ref 3). Start from the supplier’s dilution for western blotting and adjust from there.
- Wash. Three washes in TBST (ref 3).
- Secondary antibody. Incubate with an HRP- or fluorescently labeled secondary for 1 to 2 hours (refs 3, 5), then wash three times again.
- Detect and image. Develop with an ECL substrate for chemiluminescence or image the fluorescence directly. Take the longest exposure at which the strongest spot you need to measure is still not saturated, and keep the raw image file.
A run with a 2-hour primary fits in one working day. With an overnight primary it spans two.
For a DNA or RNA dot blot, the layout and the reading are the same, but the spots are probed by hybridization with a labeled nucleic acid probe instead of an antibody (ref 2).
How to read dot blot results
Read a dot blot one row of controls at a time before you look at your samples.
Start with the blank and the negative control. They show the background. A spot only counts as present if it is clearly above both, and the clearest way to decide is a threshold you set before you look: for example, signal above the blank plus three standard deviations of the blank spots. If the negative control lights up, the antibody is binding something other than your target, and the comparison between samples cannot be trusted (ref 6).
Next, the positive control and the standards. The positive control shows the antibody and detection worked. The dilution series shows where the signal stops rising in step with the amount of protein. In one study, a tubulin signal rose linearly up to about 1 microgram of lysate per spot and then flattened (ref 3). Any sample spot that is as bright as the flat part of the curve, or brighter, is outside the linear range: dilute it and run it again rather than reporting the number.
Then the samples. A darker spot means more target, inside the linear range. Compare duplicates first: if two spots of the same sample differ a lot, look at their shape before trusting either one. Uneven spots, rings and smears are covered in the troubleshooting section below.
Finally, compare samples as numbers, not by eye. A dot blot gives you a relative amount for each sample, normalized and checked against the standards. With a purified protein standard of known amount, it can also give an absolute amount (ref 4).
How to quantify a dot blot
Quantifying a dot blot follows the same logic as densitometry on a western blot:
- Define a measurement area for every spot. Use the same size and shape of area for every spot, large enough to hold the whole spot.
- Subtract background. Measure the membrane around the spots, or the blank spots, and subtract it from each spot.
- Check for saturation. Any spot whose signal has reached the detector’s maximum cannot be measured.
- Normalize. Correct each sample for how much protein was loaded. Common options are a loading control measured on a second membrane spotted with the same samples, such as tubulin (ref 3), a total protein stain of the spots, or normalization to designated control spots.
- Calibrate. Fit a curve to the standards and read each sample off it, or express each sample relative to a control sample.
- Average the replicates and report the spread.
In ImageJ this means drawing a circle around every spot by hand, measuring, copying the numbers into a spreadsheet and repeating it for the next membrane. The more spots, the more room for a circle drawn a little differently from the one before.
Dot blot troubleshooting
High background across the membrane. The usual causes are too much primary or secondary antibody, too little blocking or washing, old buffers or too long an exposure (ref 5). Dilute the antibodies, block for the full hour in fresh buffer, add washes, and shorten the exposure. PVDF gives more background than nitrocellulose and needs thorough washing (ref 5).
No signal, even in the positive control. Check that the antibody is suitable for the native or denatured protein you spotted, that the secondary matches the primary’s host species, and that no buffer used with HRP contains sodium azide, which inactivates HRP (ref 5). If the positive control works and your samples do not, the target may be below detection; spot more protein or use a more sensitive substrate.
Signal in the blank or negative control. The antibody is binding the membrane, the blocking agent or another protein in the sample. Try a different blocking agent and a lower antibody concentration (refs 5, 6). If a knockout or negative lysate still lights up, the antibody is not specific enough for a dot blot; check it on a western blot.
Uneven spots, rings or comet tails. Large drops spread and dry unevenly, leaving rings, tails or lopsided spots. Spot smaller volumes slowly, keep the tip just above the membrane, keep the membrane flat, and let each spot dry before blocking. Spots that are uneven should be re-spotted rather than measured.
Spots that run together. The spots are too large or too close. Reduce the volume, space the grid wider, or use a manifold.
Every spot looks the same. Either the spots are saturated, or the load is above the linear range so that differences are compressed. Load less protein, check the dilution series, and image at a shorter exposure.
Patchy or speckled background. Usually uneven agitation, aggregated secondary antibody or a contaminated blocking agent. Keep the membrane covered and moving during every incubation, and filter or spin the blocking agent and secondary if needed (ref 5).
Dot blots and western blots share their blocking, antibody and detection steps, so the western blot troubleshooting guide covers many of the same problems in more detail.
Quantifying dot blots with Phoretix Array
Phoretix Array is TotalLab’s software for dot blots, slot blots, cytokine and antibody arrays, ELISpot and FluoroSpot plates and other array and plate-format assays (ref 7). It turns a membrane image into a table of spot values in a few steps:
- Fit the grid. Place a measurement grid over the spots, or let Phoretix Array detect it, with built-in 96- and 384-well templates for plate formats (refs 7, 8). Every spot is measured with the same area.
- Subtract background. Choose surface minimum, edge average, negative control wells or an image rectangle, or none, and apply it the same way to every spot (ref 8).
- Normalize. Normalize to one well, or to the average of several control wells.
- Calibrate. Fit a curve to your standards, with a choice of curve fit, and read unknown samples off it (refs 7, 8).
- Call presence. Set a threshold and mark every spot present or absent across the whole array.
- Save it as a template. The next membrane with the same layout goes straight to results, and the template can be shared with colleagues and other sites so everyone runs the same analysis.
- Report. Export to CSV or a PDF report.
Phoretix Array reads TIFF, IMG, JPEG, PNG and BMP images from any imager, with chemiluminescence, fluorescence or colorimetric detection, runs on Windows, macOS and Linux, and returns results in under a minute per image. With AuditSafe it adds audit trails, electronic signatures, granular user permissions and image authenticity verification for FDA 21 CFR Part 11 and EU Annex 11 (ref 7).
“I really liked the software, it was easy to use and gave me everything I needed for quantification of dot blots. It was the first time I did this kind of experiment and this software saved me a lot of time.”
Johnny Cabrera, Novartis
If you quantified with Phoretix Array, the methods sentence reads: “Spot intensities were quantified using Phoretix Array (TotalLab Ltd, Newcastle upon Tyne, UK) after background subtraction and normalized to the [reference/positive control] spots.” Replace the bracketed part with the controls on your membrane (ref 7).
The video below quantifies a dot blot from start to finish in Phoretix Array: the grid, each background method, normalization, a calibration curve, presence and absence, and a saved protocol for the next membrane.
Quantify your next dot blot in under a minute
Open one of your own membranes in a free Phoretix Array trial: fit the grid once, subtract background, normalize to your control spots and export the numbers.
Frequently asked questions
Q: What is a dot blot?
A: A dot blot detects a protein, or a DNA or RNA sequence, by spotting the sample directly onto a nitrocellulose or PVDF membrane and probing it, without separating the sample on a gel first. It is quick, needs little sample and fits many samples on one membrane, but it cannot show the size of what it detects.
Q: What is the difference between a dot blot and a western blot?
A: A western blot separates proteins by size before probing, so you can see whether the antibody binds one protein of the expected size. A dot blot skips the gel, so it is faster, uses far less sample and handles many more samples, but any off-target binding adds to the same spot. Confirm antibody specificity on a western blot before using a dot blot to compare samples.
Q: Can a dot blot be quantitative?
A: Yes, if the antibody is specific, the spots are inside the linear range set by a dilution series, background is subtracted, each sample is normalized for loading and replicates are averaged. With a purified protein standard of known amount, a dot blot can also give absolute amounts.
Q: How much protein should I load on a dot blot?
A: Far less than on a western blot. 0.25 to 2 micrograms of cell or tissue lysate in 1 to 2 microliters per spot is a typical starting range for a single target, but the linear range depends on the antibody, so run a dilution series to find it.
Q: Should I use nitrocellulose or PVDF for a dot blot?
A: Nitrocellulose binds protein strongly and gives a lower background, which suits most dot blots. PVDF is tougher and can be stripped and reprobed, but gives a higher background and must be wetted in methanol first.
Q: How long does a dot blot take?
A: About a working day with a 2-hour primary antibody incubation, or two days with an overnight primary at 4 °C. Spotting and drying take under an hour, blocking about an hour, and the secondary antibody 1 to 2 hours.
Q: How do I check that my dot blot signal is specific?
A: Include a negative control that lacks the target, such as a knockout lysate, and a positive control that contains it, and confirm on a western blot that the antibody gives a single band of the expected size. Some suppliers also sell the blocking peptide for their antibody’s epitope: pre-incubating the antibody with it should remove a specific signal.
References
- Hawkes R, Niday E, Gordon J. A dot-immunobinding assay for monoclonal and other antibodies. Anal Biochem. 1982;119(1):142-7. doi:10.1016/0003-2697(82)90677-7
(Source for: the dot immunobinding assay, 1982.) - Kafatos FC, Jones CW, Efstratiadis A. Determination of nucleic acid sequence homologies and relative concentrations by a dot hybridization procedure. Nucleic Acids Res. 1979;7(6):1541-52. doi:10.1093/nar/7.6.1541
(Source for: dot hybridization of DNA and RNA, 1979.) - Tian G, Tang F, Yang C, Zhang W, Bergquist J, Wang B, Mi J, Zhang J. Quantitative dot blot analysis (QDB), a versatile high throughput immunoblot method. Oncotarget. 2017;8(35):58553-58562. doi:10.18632/oncotarget.17236
(Source for: dot blot developed to simplify western blotting; antibody specificity verified by western blot first; about 1 microgram per spot in 2 microliters against about 40 micrograms per western lane; tubulin linear to about 1 microgram then plateau; linear range dependent on antibody; 0.25 to 2 micrograms typical; drying 15 minutes at 37 °C or 45 minutes at room temperature; TBST rinses; 5% non-fat milk blocking; primary overnight at 4 °C or 2 hours at room temperature; secondary 2 hours; three washes; reference sample dilution curve; normalization to tubulin; agreement with western blot.) - Qi X, Zhang Y, Zhang Y, Ni T, Zhang W, Yang C, Mi J, Zhang J, Tian G. High Throughput, Absolute Determination of the Content of a Selected Protein at Tissue Levels Using Quantitative Dot Blot Analysis (QDB). J Vis Exp. 2018;(138):56885. doi:10.3791/56885
(Source for: absolute quantification against a protein standard.) - Mahmood T, Yang PC. Western blot: technique, theory, and trouble shooting. N Am J Med Sci. 2012;4(9):429-34. doi:10.4103/1947-2714.100998
(Source for: nitrocellulose and PVDF properties; wetting PVDF in methanol; blocking in 5% milk or BSA for 1 hour; BSA for phospho-specific antibodies; secondary 1 hour; causes of high background, no signal and patchy spots; sodium azide inactivating HRP.) - Ghosh R, Gilda JE, Gomes AV. The necessity of and strategies for improving confidence in the accuracy of western blots. Expert Rev Proteomics. 2014;11(5):549-60. doi:10.1586/14789450.2014.939635
(Source for: positive and negative controls for antibody specificity; knockout negative controls; epitope peptides to verify specific binding; non-specific commercial antibodies.) - TotalLab. Phoretix Array product page.
(Source for: applications; grid and templates; 96 and 384 well templates; background subtraction; normalization to one or several wells; quantity calibration; presence assignment; CSV and PDF export; file formats; detection methods; operating systems; under a minute per image; AuditSafe wording; Johnny Cabrera testimonial; how-to-report sentence.) - TotalLab. “Dot Blot Quantification Step by Step | ImageJ Alternative”, YouTube, 25 July 2025, video jsbEr3yIsZc.
(Source for: automatic grid detection; surface minimum, edge average, negative control wells, image rectangle and no background subtraction; curve fit selection; protocols.)