How to Calculate Protein Concentration from BCA Assay

1. What is the BCA Assay?

The bicinchoninic acid (BCA) assay is a copper-based colourimetric method for measuring total protein concentration in solution. It was introduced by Smith et al. (1985) at Pierce Chemical and remains, four decades on, one of the two most widely used protein quantitation methods in biochemistry and bioprocess laboratories.

The assay works by a two-step chemistry. First, peptide bonds and four amino acid side chains — cysteine, cystine, tryptophan, and tyrosine — reduce Cu²⁺ ions to Cu⁺ in alkaline solution (the biuret reaction). Second, two molecules of BCA chelate each Cu⁺ ion to form an intensely purple-coloured complex with a broad absorbance maximum at 562 nm. The colour intensity is approximately proportional to the protein concentration over the working range of the assay.

Two amino acids dominate the response. Wiechelman et al. (1988) showed that cysteine, cystine, tryptophan, and tyrosine residues account for the majority of the colour at the standard 60 °C protocol, with peptide bonds contributing increasingly at the higher temperatures used in the micro BCA variant. This is why the assay is not perfectly amino-acid composition independent — proteins with very low cysteine and aromatic content (e.g., gelatin, IgG light chains under certain conditions) can give 10–20 % lower readings than BSA-equivalent. Use a same-protein standard if you have purified material and need accuracy better than 10 %.

The chemistry runs to completion in 30 minutes at 37 °C and is robust to most detergents (Triton X-100, NP-40, SDS up to about 5 %), making it the favoured choice for crude lysates, membrane preparations, and detergent-containing buffers where Bradford fails.

2. The BCA Calculation Formula

The BCA calculation is a two-step linear regression problem. Step one fits a line to the standard curve. Step two back-calculates each unknown using that line, then multiplies by the dilution factor used to bring the sample into the linear range.

A₅₆₂ = m · [protein] + b

[protein]_sample = (A_562,sample − b) / m × DF

where m is the slope of the standard curve in absorbance units per µg/mL, b is the intercept (the residual blank-corrected absorbance), and DF is the dilution factor — e.g., DF = 10 if you diluted the sample 1:10 before assaying.

Always blank-subtract: take the absorbance of the 0 µg/mL standard (sample buffer + BCA reagent) and subtract it from every well before fitting. The intercept b after blank-subtraction is typically very small (0.00–0.05 absorbance units) and represents non-zero baseline drift; if b is large (> 0.1 AU) something is wrong with the blank or the buffer matrix.

3. Preparing the BSA Standard Curve

Prepare BSA standards from a 2 mg/mL stock. Pierce supplies sealed 2 mg/mL ampoules with their kits; alternatively, weigh out lyophilised BSA (Sigma A7906 or equivalent) and dissolve in the same buffer as your samples. Keep the stock at 4 °C and use within a week, or aliquot and freeze at −20 °C.

Standard dilution table

The recommended 9-point set spans 0–2000 µg/mL with denser spacing in the low and middle range, where most unknowns will fall.

4. Running the BCA Assay

The standard microplate protocol is the workhorse format. It uses 25 µL of sample (or standard) in a 96-well plate well, plus 200 µL of working reagent — reagent A (BCA in alkaline carbonate buffer) and reagent B (4 % CuSO₄) mixed at 50 : 1 by volume. Working reagent must be made fresh on the day; once mixed it is stable for several days at room temperature but turns greenish over time as it slowly autoreduces.

Microplate protocol step-by-step

1. Pipette 25 µL of each standard and unknown into the wells of a flat-bottom 96-well plate. Use duplicates or triplicates throughout.
2. Add 200 µL of fresh working reagent (50 : 1 reagent A : reagent B) to each well using a multichannel pipette.
3. Cover the plate with a sealing film or lid and mix on a plate shaker for 30 seconds at 600 rpm.
4. Incubate at 37 °C for 30 minutes in a plate incubator or oven.
5. Cool the plate at room temperature for 5 minutes — the absorbance is temperature-sensitive and must be read at the same temperature as the calibration.
6. Read absorbance at 562 nm on a microplate reader. Subtract the average blank (0 µg/mL) absorbance from every well, then average the replicates.
7. Fit the standard curve by linear regression and back-calculate each unknown.

The micro BCA variant (Pierce micro BCA kit, Sigma BCA1) uses a 1 : 1 sample-to-reagent ratio at 60 °C for 60 minutes and reads at the same 562 nm wavelength. It is the right choice when you need to quantify dilute samples (eluates, conditioned media, CSF) below 20 µg/mL.

5. Linear Range and Dilution Strategy

The standard BCA assay is linear from 20 to 2000 µg/mL BSA. The micro BCA assay is linear from 0.5 to 20 µg/mL. Outside these ranges the standard curve loses linearity and the back-calculation underestimates concentration. The strategy is therefore to run every unknown at multiple dilutions so that at least one falls inside the linear range.

A simple universal scheme that covers every realistic protein sample without prior knowledge:

If your sample is off-scale high (A₅₆₂ > the 2000 µg/mL standard), the calculation will give a falsely low concentration because the curve is saturating. Re-assay at higher dilution. If your sample is off-scale low (A₅₆₂ indistinguishable from blank), switch to the micro BCA variant or concentrate the sample before re-assaying.

Many protocols recommend a 4-parameter logistic (4PL) fit instead of a linear regression to extend usable range, but the bias above 2000 µg/mL is real chemistry — not a fitting artefact — and 4PL fits across the curved region still drift. For high-precision work, stick to the linear range and dilute appropriately.

6. Common Interferences and How to Handle Them

The BCA chemistry rests on Cu²⁺ → Cu⁺ reduction by peptide bonds, followed by chelation. Anything in the sample buffer that directly reduces Cu²⁺ (false positive) or chelates Cu⁺ away from BCA (false negative) will distort the assay. The Pierce technical handbook lists tolerated and interfering substances in detail; the most common bench problems are summarised below.

The TCA precipitation rescue

Trichloroacetic acid (TCA) precipitation is the universal rescue for badly contaminated samples. Add cold 100 % TCA to a final concentration of 10 % (e.g., 100 µL of 100 % TCA to 900 µL of sample), incubate on ice for 30 minutes, centrifuge at 14,000 × g for 10 minutes at 4 °C, wash the pellet twice with cold acetone, and resuspend in a clean buffer (typically 1 % SDS in 100 mM NaOH or fresh sample buffer). This removes essentially all reducing agents, chelators, lipids, and salts. The minor protein loss (typically 5–10 %) is acceptable for most quantitation purposes; for absolute accuracy, run a BSA recovery control through the same precipitation.

7. BCA vs Bradford vs A₂₈₀

BCA is one of three commonly used protein quantitation methods. The choice between them depends on your sample matrix, the precision you need, and how much sample you can spare.

Use BCA when you have a complex matrix (cell lysate, fermentation broth, membrane prep) with detergent and can tolerate a 30-minute assay. Use Bradford when you have pure protein, want a result in 5 minutes, and your buffer contains DTT or βME. Use A₂₈₀ when you have a known protein with a published or computed extinction coefficient and a clean buffer with no UV-absorbing additives — for example, when monitoring a Protein A elution from a column or quantifying a final purified IgG bulk. For deeper guidance on protein quantitation in process development, see our protein aggregation and stability guide and the inclusion bodies recovery walkthrough, both of which discuss when BCA is the right call versus when to switch to a more selective method.

Frequently Asked Questions

How do I calculate protein concentration from a BCA assay?

Build a BSA standard curve by plotting A₅₆₂ versus known BSA concentration (typically 0–2000 µg/mL across 8–9 points). Fit a linear regression A₅₆₂ = m · [protein] + b. Back-calculate each unknown as [protein] = (A_562,sample − b) / m, then multiply by the dilution factor used to keep the sample inside the linear range. Always blank-subtract the 0 µg/mL standard before fitting.

What is the BCA assay standard curve?

The standard curve plots A₅₆₂ on the y-axis against BSA concentration on the x-axis, using 7–9 BSA standards prepared by direct dilution from a 2 mg/mL stock. Standards run in duplicate or triplicate alongside unknowns on the same plate, in the same incubation. The slope and intercept are then used to convert sample A₅₆₂ to protein concentration.

What is the linear range of the BCA assay?

The standard BCA assay (microplate format) is linear from 20 to 2000 µg/mL BSA. The micro BCA variant extends sensitivity down to 0.5–20 µg/mL using more reagent and a 60-minute incubation at 60 °C. Above 2000 µg/mL the response curves and the assay underestimates true concentration.

How do I prepare BSA standards for the BCA assay?

Start from a 2 mg/mL BSA stock. Prepare standards at 0, 25, 125, 250, 500, 750, 1000, 1500, and 2000 µg/mL by direct dilution into the same buffer as your samples. Run all standards in duplicate or triplicate on the same plate, in the same incubation, as the unknowns.

Why use BCA assay instead of Bradford or A₂₈₀?

Use BCA when you need a wide linear range (20–2000 µg/mL), tolerance to detergents (Triton, SDS up to ~5 %), and amino-acid composition independence within ~10 %. Bradford is faster but detergent-sensitive and biased by basic amino acid content. A₂₈₀ needs no reagent and is non-destructive but requires a known extinction coefficient and a clean buffer.

What absorbance wavelength is used for the BCA assay?

Read at 562 nm. The Cu(I)-BCA chelate has a broad absorbance maximum around 562 nm. Any wavelength between 540 and 590 nm will work with less than 10 % loss of sensitivity if your reader lacks a 562 nm filter. Use the same wavelength for standards and unknowns on the same plate.

How long does the BCA assay incubation take?

The standard microplate protocol incubates 25 µL of sample with 200 µL of working reagent (reagent A : reagent B at 50 : 1) for 30 minutes at 37 °C. Cool to room temperature for 5 minutes before reading at 562 nm. Micro BCA uses 60 minutes at 60 °C.

What interferes with the BCA assay?

Strong reductants (DTT > 1 mM, βME > 0.01 %, TCEP > 1 mM) inflate the apparent concentration by reducing Cu²⁺ directly. Copper chelators (EDTA > 10 mM, EGTA, citrate, ammonium ions) suppress colour. Lipids and high glucose / sucrose also interfere. Fixes: dilute below threshold, TCA precipitate, switch to a Reducing Agent Compatible (RAC) BCA kit, or use a Compat-Able Protein Assay Preparation Reagent Set.

References

1. Smith PK, Krohn RI, Hermanson GT, Mallia AK, Gartner FH, Provenzano MD, Fujimoto EK, Goeke NM, Olson BJ, Klenk DC. Measurement of protein using bicinchoninic acid. Analytical Biochemistry (1985) 150(1):76–85. DOI: 10.1016/0003-2697(85)90442-7.
2. Wiechelman KJ, Braun RD, Fitzpatrick JD. Investigation of the bicinchoninic acid protein assay: identification of the groups responsible for color formation. Analytical Biochemistry (1988) 175(1):231–237. DOI: 10.1016/0003-2697(88)90383-1.
3. Walker JM. The bicinchoninic acid (BCA) assay for protein quantitation. Methods in Molecular Biology (1994) 32:5–8. DOI: 10.1385/0-89603-268-X:5.
4. Thermo Fisher Scientific Pierce. BCA Protein Assay Kit Technical Information — Instructions and Compatibility Tables. Pierce Biotechnology technical document.