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The basic recommended design features of TB laboratories to ensure appropriate biosafety are:

• Adequate ventilation and directional airflow; 6-12 air exchanges per hour (ACH) is adequate ventilation for TB laboratories.
• Ample space for the safe conduct of laboratory work and for cleaning and maintenance (Figure 1).

Figure 1: Ample and open spacing in an ideal TB lab

The Cartridge-based Nucleic Acid Amplification Test (CBNAAT) files should be archived and saved to a CD or other appropriate external media (preferably an external drive), at least once a month to ensure that no test data are lost.

Archiving tests creates copies of the test data in “gxx” files. 

Importance of Archiving and Back-up 

Archiving allows you to:

1. Back-up data to ensure it will not be lost if the computer breaks down.

Monthly maintenance tasks of the CBNAAT instrument include:

Cleaning of the Cartridge Bay and Plunger Rod

The CBNAAT manufacturer recommends monthly cleaning of the cartridge bay and plunger rod. However, if the sample load is high, this task can be carried out on a weekly basis.

Cleaning of Module PCR Slot (Figure 1)

While using the CBNAAT machine in a laboratory setting, many different causes can lead to an error (indicated on-screen as ‘ERROR’, as in figure 1).

Figure 1: Window showing how to click on “Errors” to learn more about the issue

Error Message Categories

The errors are displayed by the instrument’s software to highlight various technical issues that disrupt the test process. They are grouped into five categories by the software:

This hardware problem occurs when a cartridge is stuck inside a CBNAAT module.

Causes

• Module mechanical malfunction during the test
• Electrical failure

Solutions for releasing a stuck cartridge

Solution 1: Verify that the module door is not open. Gently try to open the module door.

Solution 2: Try to remove the cartridge from the software.

Common hardware problems that can occur while using the CBNAAT machine and their recommended solutions are listed below.

The CBNAAT software displays the result as Rif Indeterminate when the early Cycle threshold (Ct) value is greater than 35, which suggests that the bacillary load in the specimen is very low.  

The software cannot calculate the difference in the Ct values, hence the result is declared 'Indeterminate'. ​ 

This can be seen in the figure below.

Figure: Ct>35 shows Rif Indeterminate Result

Resources

In Cartridge-based Nucleic Acid Amplification Test (CBNAAT) assay, the basis for detection of rifampicin resistance is the difference between the first or early Cycle threshold (Ct) and the last or late Ct value, of M. tuberculosis-specific probes. Or the difference between probes with the maximum Ct value and minimum Ct value is greater than four.

The Cartridge-based Nucleic Acid Amplification Test (CBNAAT) software generates the results from the measured fluorescence signals and embedded calculation algorithms. In addition to qualitative results, the software provides semi-quantitative results, which provide information about the bacterial load in the specimens tested.

Semi-quantitative result interpretation is based on Cycle threshold (Ct) values of Mycobacterium tuberculosis(MTB) specific targets (Figure 1).

The comparison of the Acid-Fast Bacilli (AFB) and Light Emission Diode (LED) Fluorescence Microscopy is tabulated below: