Rapid escalation of falsely positive Xpert results in TB laboratories
Writing a TB lab procedure for rapid escalation of falsely positive Xpert results is more than repeating a specimen until it becomes negative. A positive molecular result may trigger treatment, respiratory precautions and contact investigation. The procedure should distinguish a suspected false positive from a confirmed error, prevent unreliable reporting and preserve every decision while urgent clinical action continues.
An effective document defines triggers, assigns authority, protects specimens and data, and states when results must be escalated. It also fits Australian requirements: tuberculosis is notifiable, laboratories commonly use NATA-accredited quality systems, and international movement of specimens or devices can invoke biosecurity and customs controls. Response must be rapid without treating every unexpected positive as harmless or every first positive as false.
Set the trigger and protect people at risk
Use “suspected false-positive Xpert result” until evidence supports a firmer conclusion. Activate the SOP for an unexpected positive, a repeat that conflicts with clinical information, several unexpected positives linked by a lot, extraction batch, instrument or work area, or any event casting doubt on a reported result. Include specimens with suspected collection or transport problems and sample types not validated for the assay, because these conditions require investigation rather than automatic acceptance or rejection.
Immediately stop work in the affected zone, retain remaining specimens and cartridges, and alert the supervisor and result authoriser. If a result has been released, telephone the treating clinician and infection-control team rather than waiting for written escalation. Where policy permits, label the preliminary report as under investigation while keeping the original result legible and attributable. Never resolve the issue through an informal correction that disappears from the electronic record.
Name a primary responder and backup for every shift. Define each escalation target in minutes or hours rather than using vague terms such as “promptly”. Establish after-hours routes through the on-call microbiology lead and state or territory TB program. Record the time of each notification and align the process with the laboratory’s validated reporting, non-conformance and document-control procedures.
Build a controlled repeat-testing workflow
The repeat must be independent, not simply another read of the same reaction. If specimen volume remains acceptable, prepare a new aliquot in a clean extraction area and test it on a new single-use cartridge, with the assay’s internal and sample-processing controls interpreted as required. Follow the manufacturer’s instructions, validated contamination-control procedures and a unidirectional workflow. Separate pre-amplification and post-amplification activities, change gloves between specimens and use validated DNA decontamination for equipment and spills.
Teams adapting global quality-system materials can use the GLI Quality Tool’s user instructions to align checklists, responsibilities and evidence with the local SOP. The repeat-testing section should identify who releases the specimen, who performs extraction and testing, which instrument is used, how controls are assessed and where aliquots are stored. Testing on a separate instrument can help localise a problem, but it must not bypass required quality controls.
Interpret the repeat alongside control results, the original finding and clinical information. A negative repeat does not automatically prove that the first result was false, while another positive does not identify the cause by itself. Review specimen type and condition, transport, operator records, environmental cleaning, reagent lots and instrument performance. Retain discrepant cartridges, quarantine implicated materials and use a separately prepared specimen for any onward reference testing.
Contain contamination and build defensible records
Containment begins with people, equipment and material. Close the affected bench or room according to the local risk assessment, restrict access to trained staff and segregate remaining specimens, extracts and single-use cartridges. Clean and decontaminate surfaces, pipettes, racks, instruments and waste routes with validated products and contact times. Replace exposed reagent stocks, document every action and release the area only after managerial authorisation.
The investigation record should contain patient and specimen identifiers, result and control history, collection and transport details, operator and equipment identifiers, reagent and cartridge lots, dates, cleaning evidence, calls made, repeat results, reference reports and the final risk decision. Preserve raw reports, instrument audit trails, maintenance records and electronic communications. Never overwrite the original finding; issue an authorised amended report, explain the reason and identify who approved the change.
Once immediate risks are controlled, conduct a structured root-cause analysis. Distinguish laboratory error from true infection, specimen contamination, carry-over, reagent or instrument problems and pre-analytical issues. Check whether other patients used the same lot, cartridge box, extraction batch or cleaning cycle. Assign owners and due dates to corrective and preventive actions, then verify effectiveness through later QC data, audits or simulated escalations. A closed case without effectiveness evidence remains incomplete.
Coordinate clinical, reference and public health action
Clinical communication is part of the technical response. Explain the preliminary concern without offering an unsupported conclusion, document the clinician’s decision and arrange further TB assessment when the result remains uncertain. Infection-control staff may need immediate precautions because delaying action can cause greater harm than briefly retaining them. The laboratory must also meet applicable notification duties and follow local rules for corrected results and health-record notifications.
Use the patient’s jurisdiction to contact the appropriate specialist service, such as the Melbourne-based Victorian TB service, the NSW TB Program in Sydney, Queensland’s Brisbane-based TB service or the WA TB program in Perth. State why the result is disputed, what repeats and controls have been performed, and whether infectious material is being sent. A reference laboratory can organise independent extraction, an alternative molecular assay, culture or other confirmatory testing, subject to specimen quality and its validated methods.
For material leaving Australia, use an approved international specimen-courier pathway rather than ordinary post or passenger baggage. Australian Customs and Department of Agriculture, Fisheries and Forestry biosecurity requirements, destination-country permits, packaging validation and carrier dangerous-goods rules may apply. Confirm that the receiving laboratory is authorised and willing to accept the specimen before dispatch, and retain all permits, declarations and shipment records. Domestic referrals must still follow their state transport and packaging protocol.
Recommendations for putting the procedure into practice
A useful comparison separates a local repeat from a full investigation and direct reference escalation. Choose the route according to patient risk, specimen availability, the strength of the concern and whether other results may be affected.
| Situation | Immediate pathway | Required evidence |
|---|---|---|
| One unexpected positive; adequate specimen | Repeat a fresh aliquot with full controls; communicate concern | Aliquot, controls, both reports, authorised notification |
| Conflicting repeat or poor original specimen | Stop the affected batch; seek expert review | Quarantine, specimen history, reagent and instrument traceability |
| Cluster linked by time, lot or area | Suspend the workflow or lot; arrange parallel reference testing | Cleaning, lot mapping, environment, patient impact |
| Result already used for care | Expedite clinical and public-health contact; preserve and amend records | Call, decision, notification and amendment record |
Cost the response before approval, including staff time, cartridges, courier transport, repeat testing, downtime, clinician communication and record review. The GLI Quality Tool’s quality improvement costing guidance can help create a transparent local estimate. Australian quotations may come from public-health tenders or authorised laboratory distributors, so document the supplier, cartridge model, pack configuration and service assumptions rather than comparing cartridge prices alone.
Before release, ensure the SOP contains these essentials:
- A one-page trigger-and-contact sheet with named roles, backup contacts and service targets.
- A clean-zone repeat-testing sequence that prohibits cartridge reuse or returning opened material to stock.
- A quarantine and release process for specimens, cartridges, reagents, instruments and affected reports.
- A communication tree covering the result authoriser, clinician, infection-control lead, laboratory manager and TB program.
- A review cycle that tests effectiveness through control data, audits, complaints and a documented simulation.
Adopt the procedure this quarter, brief every shift and laboratory location, and begin a simulated escalation with a disputed result. Assign an owner to collect the evidence, close every action and report the pilot outcome to laboratory leadership and the relevant TB program.