Coagulation Analyzer Carryover: A Practical Check for High-to-Low Sample Sequences

Automated coagulation analyzers are designed to process many samples with minimal contamination between reactions. Even so, carryover deserves specific attention during installation, method verification, application changes, and investigation of unusual result sequences. A small amount of material transferred from a high-concentration or strongly reactive sample can affect the next low sample, particularly in sensitive quantitative assays.

Carryover is not only a probe-washing question. It can involve the sample probe, reagent probe, mixing system, cuvette path, wash station, tubing, or software sequence. The practical goal is to determine whether a challenging sample leaves enough residual material to cause a meaningful change in subsequent results under the actual laboratory workflow.

Define the risk for each assay

The same carryover design does not fit every coagulation test. For D-Dimer and FDP, a very high result followed by a low sample can be an obvious high-to-low challenge. Antithrombin and fibrinogen require consideration of method response, concentration range, dilution, and application. PT, APTT, and thrombin time may involve highly prolonged or non-clotting samples rather than a simple concentration gradient.

The laboratory should identify which materials could contaminate the next reaction and what result region is vulnerable. Carryover from a reagent or calibrator may follow a different path from patient-sample carryover. Open reagent containers, shared probes, onboard dilution, reflex testing, and rerun sequences should be included in the risk review.

Choose high and low materials that challenge the system

A useful experiment needs a high material capable of creating a realistic contamination challenge and a low material whose result would reveal a small transfer. The high sample should be within a range the analyzer may encounter, while still stressing the wash process. The low sample should be stable and close enough to an important reporting region that contamination can be detected.

Patient pools, suitable control materials, or other validated samples may be used according to laboratory policy. Manufactured controls can behave differently from patient plasma, so the choice should match the question. Samples should be homogeneous, sufficiently voluminous for repeated measurements, and free from obvious instability. Extremely artificial materials may produce a result that does not represent routine operation.

Use an ordered measurement sequence

A common design alternates repeated high measurements with repeated low measurements and compares the first low result after high samples with later low results. The exact sequence and calculation should be predefined. Replicates help separate carryover from normal imprecision, while a low-low baseline shows how the low material behaves without a preceding high challenge.

The sequence must reflect the analyzer’s actual routing. If the instrument automatically uses separate cuvettes, performs onboard dilution, or assigns different probe washes, the study should preserve those actions. Manually rearranging samples in a way the analyzer would never process may answer the wrong question.

Set acceptance criteria before reviewing data

Carryover can be expressed as an absolute difference, percentage, or ratio relative to the high-low separation. The laboratory should choose an approach appropriate to the assay and define what difference would be operationally or clinically meaningful. The acceptance limit should account for method imprecision near the low level.

A statistically detectable change is not automatically important, but a small numerical change may matter near a cutoff or reporting boundary. For D-Dimer, unit and threshold context are essential. For antithrombin or fibrinogen, the effect should be considered against analytical performance and decision regions. Criteria should not be selected after seeing whether the analyzer passes.

Investigate patterns, not just the final percentage

If the first low result is shifted and later low replicates return toward baseline, the pattern supports a sequence-related effect. If all low replicates are unstable, imprecision, sample mixing, evaporation, or calibration may be more likely. If results drift throughout the run, temperature or material stability deserves review.

The laboratory should also examine flags, reaction curves, aspiration errors, wash alarms, and cuvette handling. A reported numerical result can hide an abnormal curve that points to contamination or optical interference. Investigation should record the exact sample order, probe path, dilution events, reagent positions, and maintenance state.

Distinguish sample carryover from reagent contamination

Sample carryover transfers material from one specimen to the next. Reagent contamination may occur when a shared probe carries one reagent, calibrator, or cleaning solution into another container or reaction. The troubleshooting approach differs. Repeating only the high-low patient sequence may not reveal a reagent-probe problem.

When several assays shift after a new reagent is loaded, the laboratory should inspect probe assignment, wash settings, bottle placement, caps, dead volume, and application configuration. Contaminated reagent bottles may continue producing errors even after the instrument wash system is restored. Reagent replacement should follow evidence rather than serve as the automatic first step.

Connect the study to maintenance

Carryover performance depends on probe surfaces, wash solution, tubing, pumps, alignment, water quality where applicable, and maintenance frequency. A new analyzer may pass installation checks but develop risk later if cleaning is incomplete or consumables are substituted. Trending wash alarms and reviewing difficult sample sequences can reveal deterioration before routine QC is affected.

After probe replacement, wash-system repair, major maintenance, or application modification, focused rechecking may be appropriate. The scope should match the change. Laboratories should also confirm that the cleaning solution and consumables are approved for the analyzer and used at the intended concentration.

Build protective workflow rules when needed

If a meaningful effect is confirmed, the response may include enhanced wash, rerunning the low sample, separating specific assays, changing sample order, adjusting application settings under manufacturer control, or servicing the instrument. The laboratory information system or operating procedure should make the rule visible to staff.

Distributor and manufacturer support is most effective when the escalation includes raw results, sample sequence, reaction curves, lots, application version, maintenance records, and the acceptance rule. For OEM or localized reagents, carryover evaluation should be part of analyzer application development rather than deferred until a customer complaint.

A well-designed carryover check is narrow, but it tests an important assumption: that the current result belongs only to the current sample. By challenging realistic high-to-low sequences, evaluating patterns, and linking findings to maintenance and workflow, laboratories can protect low results from contamination that routine QC may not reveal.