Biotech & Diagnostics

A Positive Control Should Occasionally Worry You: Designing Challenge-Led Controls for UK Immunoassays

A positive control that passes every run can still be a poor control. That is not a paradox; it is a design problem. In many UK antibody laboratories, the positive control has become a ceremonial well: present on every plate, expected to produce a signal, and rarely asked whether that signal resembles the samples the assay is meant to judge.

The result is a particular kind of false confidence. The run is technically complete, the acceptance criteria are met, and the release decision proceeds. Yet the control may never have been placed under meaningful stress. It may have been spiked at a concentration far above the clinical decision point, prepared in a buffer that flatters the antibody pair, or stored in a matrix that shares little with the sera or plasma arriving from UK clinics. When such a control agrees with the assay, it is not confirming much beyond the pipette tip and the plate reader.

The comfort of a green well

A positive control should do more than demonstrate that the detection system can generate light, colour or a line. It should challenge the assay at a point where a wrong answer would matter. That distinction is easy to lose when controls are inherited from an earlier development phase and carried forward because they have always passed.

In accredited UK clinical laboratories, the language of quality management encourages documented acceptance criteria and trend review. Those are necessary disciplines. They do not, by themselves, ensure that the control is commutable, appropriately placed or connected to the decision the assay supports. A control can be well documented and still be analytically easy.

Three jobs behind one label

The first practical step is to stop using a single label for several different control functions. A system suitability control, sometimes called an assay control, confirms that the analytical system is operating within its expected range. It is a check on reagents, instrumentation and technique. It need not resemble a patient sample, and it should not be treated as evidence of clinical performance.

A process control monitors the steps that sit between sample receipt and result. If extraction, dilution, wash or neutralisation is part of the workflow, the process control should be sensitive to failure in those steps. Its matrix and preparation should reflect the process being monitored, not merely the detector.

A decision-point control sits closer to the clinical question. It is prepared at or near the concentration at which the assay's interpretation changes, and it should be commutable with the sample types the laboratory receives. Its purpose is to show that the assay can still distinguish a result on one side of the decision boundary from a result on the other. That is a different demand from confirming that a high spiked sample produces a high signal.

The concentration trap

Spiking a control at a conveniently high level is common because it produces a robust signal and forgiving acceptance limits. It can also conceal exactly the failures that matter. An antibody pair that has lost sensitivity at the low end may still perform well when challenged with a concentration several times above the cut-off. A control that sits in the saturated part of the curve will not reveal the drift that affects borderline samples.

A more defensible design places controls at concentrations that reflect the intended use. For a qualitative assay, that means at least one control close to the cut-off, with acceptance criteria that reflect the uncertainty of the method. For a quantitative assay, it means covering the clinically relevant range rather than only the upper end. The exact placement should be justified in the control strategy, not chosen because it gives a comfortable coefficient of variation.

Matrix is part of the control

The diluent is not a neutral backdrop. It can alter antibody binding, enzyme activity, nonspecific interactions and the recovery of the analyte. A control prepared in a generic buffer may pass while a serum sample from a patient with a different protein profile behaves quite differently. This is the commutability problem, and it is central to whether a control tells you anything useful about clinical samples.

Whenever possible, controls should be prepared in a matrix that resembles the intended sample type, or commutability should be demonstrated against that matrix. Residual clinical samples can be valuable for this purpose, subject to local governance, consent arrangements and anonymisation. The point is not to demand patient material for every run; it is to ensure that the control's relationship to clinical samples has been examined rather than assumed.

An audit that asks harder questions

A control-design audit can begin with a simple inventory. For each control, record its stated purpose, concentration, matrix, preparation method, storage conditions and acceptance criteria. Then ask whether those choices are consistent with the assay's intended use.

Useful questions include:

• Is this control verifying the system, the process or a decision point?
• Where does it sit relative to the clinical cut-off?
• Is the matrix representative of the samples the laboratory receives?
• Has commutability been assessed, or merely asserted?
• Are acceptance limits based on analytical performance or on historical convenience?
• Does the trend review detect gradual drift, or only outright failure?

The answers should be documented and reviewed at defined intervals. A control that has not been reconsidered since development is not necessarily wrong, but it is not automatically right either.

Documentation for IVD development

For UK IVD development, the control strategy should be visible in the technical documentation. That means a clear rationale for control selection, preparation records for spikes and pools, evidence of matrix suitability, acceptance criteria and the rules used to judge trends. If a control is intended to support a performance claim, its relationship to the claimed sample type should be traceable.

This is not bureaucracy for its own sake. It is the difference between a control that can be defended in a technical review and one that simply occupies a well.

A control should occasionally worry you

The test of a positive control is not whether it agrees with the assay. It is whether it would disagree when something meaningful has gone wrong. A control that never raises a question may be well behaved, or it may be too easy. UK antibody laboratories should be willing to make that distinction explicit, separate the functions of their controls, place them where decisions are made, and document the reasoning. A control that earns its place is not one that always passes; it is one whose passing means something.