Most ELISA reports live or die on a single number: R². It is comforting, it is familiar, and it is not enough. A curve with R² = 0.99 can still be built on a collapsed dynamic range, an EC50 that sits outside the standards, or a lower asymptote that is indistinguishable from the blank. None of that shows up in R² — but all of it silently biases the sample concentrations you report.

This post is a practical checklist of the metrics that actually decide whether an ELISA run is trustworthy, and how Labreador surfaces them automatically through the new Method Health Panel so you do not have to compute them by hand.

The five checks that matter more than R²

1. Dynamic range (ΔOD)

The difference between your highest and lowest standard OD is the "room" your assay has to discriminate concentrations. If the top standard reads 1.8 and the bottom reads 1.5, the whole assay is trying to resolve samples inside a 0.3 OD window — smaller than the plate-reader noise on many instruments.

≥ 1.5 OD — ideal, comfortable resolution across the curve
0.5–1.5 OD — marginal, low-end samples become noisy
< 0.5 OD — the assay cannot separate concentrations; re-run

A shrunken ΔOD usually points to degraded conjugate, under-developed substrate, or coating that has lost activity.

2. Signal-to-blank ratio (S/B)

S/B is the top standard OD divided by the blank OD. It tells you how far your signal sits above the background. A high R² on a curve with S/B = 2 is meaningless — half of what you are fitting is noise.

≥ 10× — clean assay
3–10× — marginal, expect poor LLOQ
< 3× — background dominates; results are not defensible

Low S/B typically means the plate was not washed thoroughly, or the blocking step was skipped or under-optimized.

3. EC50 inside the standards range

For 4PL and 5PL fits, EC50 (parameter C) is the inflection point of the sigmoid — the single best-constrained part of the curve. If EC50 falls outside your highest or lowest standard, the model is extrapolating the shape of the curve, and the asymptotes A and D become guesses rather than measurements.

An EC50 outside the standards is a strong signal that your dilution series does not span the analyte's real dynamic range. The fix is usually to add a higher or lower standard, not to accept the curve.

4. EC50 confidence-interval width

Even when EC50 lands inside the range, its 95% CI tells you how well the data actually constrain it. A ratio of upper/lower CI bound is a clean summary:

≤ 3× — EC50 is well-identified
3–10× — parameter is loose; interpolation near the middle is uncertain
> 10× — the curve is essentially under-defined; different EC50 values fit almost equally well

Wide CIs usually come from too few standards around the inflection region, or from replicates with high CV%.

5. R² — as a floor, not a ceiling

R² still matters — but as the last check, not the first. Labreador uses a three-tier gate: ≥ 0.98 ideal · 0.95–0.98 marginal · < 0.95 fail. Think of it as a floor that rules out obvious disasters, while the four checks above catch the silent ones.

Recovery: the honest test of your calibration

The metrics above describe the curve. Standards back-calculation describes what the curve does to real data. Labreador plugs each standard's OD back through the fit and reports the interpolated concentration versus the known one — the Recovery % and Bias %.

Following FDA/EMA bioanalytical guidance, the target is:

80–120 % recovery for non-anchor standards (25 % at the LLOQ)
≥ 75 % of standards must pass — the "75/25 rule"

If your R² is 0.99 but three standards recover at 140 %, your curve is wrong in a way R² will never tell you. Recovery is the ground-truth check.

How Labreador's Method Health Panel puts it together

Rather than making you compute five diagnostics by hand after every run, Labreador aggregates them into a single collapsible Method Health Panel on the ELISA results screen:

Each check gets a pass / warn / fail status with the numeric value, the target range, and a plain-language explanation of what a failure means for your data.
The panel is diagnostic only — it never silently modifies your fit or interpolation. You stay in control of every decision.
A top-line verdict (Healthy · Review recommended · Method issues detected) gives you an at-a-glance summary you can screenshot for a lab notebook or QC log.

Combined with Standards Back-Calculation, the 3-tier R² gate, and the ability to exclude individual standards from the platemap, it turns "does the curve look OK?" into a defensible, reproducible answer.

A workflow that raises method quality, not just report quality

The point of these checks is not to fail more runs — it is to catch method problems while you can still fix them:

Run the plate, load results into Labreador.
Open the Method Health Panel — if ΔOD or S/B fail, the problem is in the lab, not the fit. Re-run before spending time on interpretation.
Check the Recovery table — if the 75/25 rule fails, exclude the offending standards, refit, and treat samples in that concentration range as semi-quantitative.
Only then trust the sample concentrations, LLOQ, and ULOQ.

R² is a useful summary. It is not a quality gate. Dynamic range, S/B, EC50 placement, CI width, and recovery are — and having them one click away turns ELISA from a black box into a method you can defend.

Try the Method Health Panel in the ELISA module. Every check is documented in-app under the Help (book icon) button.

Beyond R²: What to Actually Check for ELISA Method Quality