Writing a Reliable SOP for TB Line Probe Assays

Writing a TB lab SOP for using molecular line probe assays on smear-negative samples requires more than describing the assay steps. The document must explain when testing is appropriate, how a low-bacillary-load specimen is handled, what controls are required, and how an uncertain result affects patient care and public health action.

Smear-negative respiratory specimens create particular risks. A negative auramine or Ziehl–Neelsen result does not exclude tuberculosis, while the reduced organism load can lower molecular sensitivity and increase the impact of inhibitors, contamination and poor specimen quality. A well-designed standard operating procedure (SOP) makes those limitations visible instead of allowing users to treat every result as definitive.

For laboratories in Australia, the procedure should also fit local accreditation, referral and reporting arrangements. A metropolitan service in Sydney or Melbourne may send complex testing to a state reference laboratory, while a remote service in the Northern Territory or Western Australia may manage longer transport times and limited on-site culture capacity. The SOP needs to work in both environments.

Define The Assay’s Intended Use

Start by stating exactly what the assay is intended to detect. A molecular line probe assay (LPA) may identify Mycobacterium tuberculosis complex and mutations associated with resistance to rifampicin, isoniazid or selected second-line medicines, depending on the kit. The SOP should name the assay, instrument or hybridisation platform, specimen type, extraction method and version of the manufacturer’s instructions for use.

The scope must distinguish between direct testing of a respiratory specimen and testing of a cultured isolate. Many LPA platforms have stronger evidence for smear-positive material or cultured growth than for smear-negative sputum. Direct testing of a smear-negative sample should occur only when the manufacturer permits it and the laboratory has completed a documented local validation. If that evidence is absent, the SOP should direct staff to culture, an appropriate validated NAAT, or referral testing rather than implying that a negative LPA excludes tuberculosis.

A short decision pathway can prevent inappropriate testing. Include the smear result, clinical urgency, previous treatment, known drug resistance, specimen volume and whether an initial rapid molecular test has detected M. tuberculosis. The quality roadmap can help laboratories place this SOP within broader requirements for personnel, equipment, documentation, assessment and continual improvement.

Build A Specimen And Safety Workflow

The pre-analytical section should specify acceptable specimens, minimum volume, labelling, transport conditions, rejection criteria and the maximum time between collection and processing. Sputum should be collected in a sterile, leak-resistant container, with patient identifiers matching the request form. Saliva, dried material, leaking containers and unlabelled specimens require defined actions rather than informal decisions at the bench.

For smear-negative material, the SOP should state whether the laboratory uses direct sputum, a concentrated sediment after digestion and decontamination, or material recovered from culture. It should describe how much specimen is reserved for microscopy, culture and other molecular testing. A low-volume specimen may need urgent discussion with the requesting clinician or public health unit before it is consumed.

TB processing can generate infectious aerosols. Include the required biological safety cabinet, respiratory protection and centrifuge controls, as well as decontamination, waste disposal and spill response. The document should identify which steps occur in the containment laboratory and which occur in a clean amplification area. Unidirectional workflow and separation of pre-amplification and post-amplification activities are particularly important for preventing amplicon contamination.

Australian laboratories should account for practical transport conditions. A specimen travelling from Alice Springs, Broome or a regional Queensland service may face delays that are uncommon in a large Melbourne laboratory. The SOP can specify escalation for delayed transport, temperature excursions and insufficient volume, while linking these events to the laboratory’s incident and non-conforming-work process.

Describe The Analytical Procedure Clearly

The procedure should be written so that a trained scientist can follow it without relying on memory or undocumented local customs. Set out specimen preparation, extraction or lysis, reagent preparation, strip identification, amplification, hybridisation, washing, band development and result capture in the order performed. Include allowed reagent storage temperatures, expiry checks, lot numbers, equipment settings and the action required when a kit component is missing or damaged.

Smear-negative testing needs especially clear contamination controls. Describe how samples are batched, how positive and negative controls are positioned, how pipettes are assigned, and how work surfaces are cleaned. A negative extraction control helps identify contamination introduced during processing, while assay-specific controls confirm amplification and detection performance. The SOP should state whether an invalid strip can be repeated and when a new specimen or referral is required.

Do not reproduce the manufacturer’s instructions without adding laboratory-specific controls. Record the operator, date, specimen identifier, kit lot, run identifier and instrument or thermocycler used. A secure image or electronic record of the strip can support review, troubleshooting and external assessment. The downloadable materials available through the GLI resource can assist with organising supporting forms, checklists and controlled documents.

Training requirements should be linked to each critical task. A new staff member may be competent at sputum reception but not at strip interpretation or contamination investigation. A role-based matrix can help the laboratory document supervised practice, direct observation, competency assessment and re-assessment after a significant method change.

Set Rules For Interpretation And Reporting

The result interpretation section should use the assay’s official band patterns and mutation rules. It should explain how to classify a valid susceptible pattern, a resistance-associated pattern, an indeterminate result, a missing band, mixed pattern or test failure. Avoid translating a mutation pattern into a final clinical statement unless the assay’s validated interpretation and local reporting policy support that wording.

For a smear-negative specimen, a negative LPA result should be reported with an appropriate limitation. It may mean that M. tuberculosis was not detected in the tested material, but it cannot reliably exclude disease when the organism burden is low. The report should indicate whether the specimen was suitable for direct testing and recommend culture or another validated test where clinically appropriate.

Resistance findings require prompt communication. The SOP should identify who contacts the treating team, infectious diseases service, public health unit or reference laboratory, and within what timeframe. A suspected rifampicin-resistant or multidrug-resistant result should not wait for routine batch reporting if immediate clinical action is needed. The procedure should also state when confirmatory testing, sequencing or phenotypic drug susceptibility testing is required.

Use comments that are specific rather than alarming. For example, an indeterminate result may reflect low template concentration, inhibition, degraded material or a technical failure. Repeating the assay from the same extract may be reasonable in some circumstances, but repeated testing of a poor specimen may waste time. The SOP should define the escalation route and preserve the original data for review.

Maintain Quality Across The Service

Quality indicators should cover the entire pathway, not just the percentage of successful strips. Useful measures include specimen rejection, invalid and indeterminate rates, contamination events, turnaround time, repeat testing, amended reports and agreement with referral results. Review trends by specimen type and collection site; a high invalid rate from one regional service may indicate transport or collection problems rather than an analytical fault.

The SOP should include internal quality control, external quality assessment or proficiency testing, lot-to-lot checks, equipment maintenance and document control. Every revision needs an approval date, version number, authorisation record and defined implementation date. When a kit, extraction method, thermocycler or reporting rule changes, perform a risk assessment and document revalidation before routine use.

NATA accreditation and ISO 15189-aligned practices make traceability particularly important in Australia. The laboratory should retain validation evidence showing how smear-negative specimens were assessed, including detection limits, precision, inhibition, contamination risk and comparison with an established method. Procurement should also account for the local market: kit availability through Australian distributors, cold-chain delivery, lot continuity, service support and the time needed to obtain replacement reagents.

The following comparison can help define the SOP’s pathway without presenting one method as suitable for every patient:

Testing approach	Typical role	Strengths	Key limitation for smear-negative samples
Direct LPA	Rapid resistance investigation when validated	Can identify resistance-associated mutations quickly	Low organism load may produce invalid, missed or incomplete patterns
Culture followed by LPA	Testing a confirmed isolate	More reliable template and broad confirmatory value	Slower; requires culture facilities and safe referral arrangements
Rapid NAAT	Initial detection of TB and selected resistance markers	Fast and useful when bacillary load is low	May provide less resistance information than a full LPA
Phenotypic susceptibility testing	Confirmation or extended drug panel	Measures growth response to medicines	Slow and technically demanding
Referral or sequencing	Complex, discordant or unusual results	Supports resolution of mixed or uncommon resistance	Depends on transport, funding and reference-laboratory capacity

A controlled SOP should finish with appendices for the worksheet, rejection codes, result comments, contact list, troubleshooting pathway and competency record. Keep the working copy accessible at the bench, while ensuring obsolete versions are removed. Review the document after incidents, audit findings, assay changes and updates to national or manufacturer guidance.

Put the procedure into controlled use with trained staff, documented competency and scheduled review. A clear pathway protects patients, supports rapid resistance management and gives Australian TB laboratories a defensible record of how smear-negative molecular results were produced and interpreted.