Standardized Application of Mycobacterium tuberculosis Complex DNA Detection Kit (Real-Time Fluorescence PCR Method) in the Laboratory

Sample Collection and Transportation Requirements

When using the Mycobacterium tuberculosis complex nucleic acid (DNA) detection kit (real-time fluorescence PCR method), sample quality directly affects the test results. It is recommended to collect sputum from patients after deep coughing in the early morning, with a specimen volume generally required to be 3–5 mL. For extrapulmonary samples such as pleural effusion and cerebrospinal fluid, they should be collected in sterile containers and sent to the laboratory as soon as possible. During transportation, low temperature (2–8°C) should be maintained, and repeated freeze-thaw cycles should be avoided. After the laboratory receives the samples, the receipt time and appearance characteristics should be recorded, and unqualified samples (such as saliva-like sputum, obvious food residues, etc.) should be promptly returned or noted. The DNA extraction procedure must strictly follow the kit instructions, using the matching lysis buffer and purification columns to ensure the integrity and purity of nucleic acids and remove PCR inhibitors. It is recommended to operate in a biosafety cabinet to prevent aerosol contamination.

Amplification and Detection Procedure

Before amplification, all components of the kit should be fully thawed, mixed thoroughly, and centrifuged to avoid air bubbles. When preparing the PCR reaction mixture, attention should be paid to changing pipette tips to avoid cross-contamination. After adding the template DNA to the reaction tube, briefly centrifuge and place it into the real-time fluorescence PCR instrument to run the preset program. Common programs include pre-denaturation, cyclic amplification, and cooling stages. The instrument automatically collects fluorescence signals and generates amplification curves. The laboratory should regularly calibrate the temperature accuracy and fluorescence channel sensitivity of the fluorescence PCR instrument and record instrument operation logs. Each experiment must use the positive control (with a defined Ct value range) and negative control provided by the kit. If the Ct value of the positive control exceeds the manufacturer's set range, or if the negative control shows a typical amplification curve, the experiment is invalid, and the cause should be analyzed before retesting.

Result Interpretation and Reporting Specifications

Result interpretation is based on the Ct threshold and endpoint fluorescence intensity in the instructions. Generally, when the sample amplification curve shows a typical S-shape and the Ct value is below the cutoff value, it is judged as positive; otherwise, it is negative. However, attention should be paid to the repeatability of weakly positive samples (Ct values close to the cutoff), and re-extraction or retesting is recommended for confirmation. The report content should include the detection method, result (positive/negative), Ct value (if available), and remarks (such as sample quality assessment). Clinical reports should not contain the words "confirmed" or "excluded," but should be expressed as "Mycobacterium tuberculosis complex nucleic acid detection positive/negative." For situations where the test results are inconsistent with clinical findings, the laboratory should communicate with the clinician and, if necessary, recommend retesting or verification by other methods.

Common Interfering Factors and Countermeasures

The Mycobacterium tuberculosis complex nucleic acid detection kit (real-time fluorescence PCR method) may be interfered with by various factors. Heparin, hemoglobin, bile salts, etc. present in samples can inhibit the PCR reaction, leading to false negatives; aerosol contamination or residual amplification products can cause false positives. Laboratories should use a UNG enzyme (uracil-N-glycosylase) system to prevent amplification product contamination and set up a no-template blank control in each batch. In addition, gene mutations in the primer-probe binding region may lead to amplification failure, resulting in false negatives. It is recommended that laboratories pay attention to the genotypes of prevalent strains in their region and communicate with the manufacturer if necessary. Improper storage of the kit (such as repeated freeze-thaw cycles, high-temperature exposure) can also reduce detection sensitivity, and it should be strictly stored and used according to the conditions specified in the instructions.