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Error or indeterminate DST results are reported when certain conditions occur that may affect the test. These results are designated by the alphabet ‘X’ followed by a code. Following two types of such errors are encountered:

  1.        X200 = Occurs when the system cannot detect a sufficient indication of growth in the Growth Control tube in the specified protocol time (i.e., 13 days for  SIRE or 21 days for PZA) and does not provide an interpretation of the AST set results. Often a result of too little inoculum, nonviable organisms, or a slow-growing drug-resistant strain. 

    Troubleshooting: In this case, repeat testing with a pure culture of the isolate is recommended.

2.  X400 = Occurs when the system detects indications of sufficient growth in the growth control (GC) tubes in less than four days. It may be a result of a possible contamination or over-inoculated tube. 

    Troubleshooting: Check the GC tube for turbidity, prepare a ZN smear and subculture to a blood agar (BA) or brain heart infusion (BHI) agar plate     and interpret as below:

 

Condition

Interpretation

Control is visually turbid. 

ZN – positive

BAP/BHI– growth

Culture positive for AFB and contaminated.

Verify original culture was MTB, purify if MTB, and repeat the test.

Control has no visual contamination.

ZN – positive 

BAP/BHI – no growth

Over-inoculation; Repeat the test

Control is either visually turbid or clear

ZN – negative

BAP/BHI – growth

Culture contaminated.

Verify original culture was MTB and repeat test.

Resources

  1. Mycobacteriology Laboratory Manual

  2. Revised National TB Control Programme Training Manual for Mycobacterium tuberculosis Culture & Drug susceptibility testing, Central TB Division, Ministry of Health and Family Welfare, GOI

Assessment

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Answer 1​

Answer 2​

 Answer 3 Answer 4

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X400 occurs if the system detects growth in the GC before four days.

True

False

    

1

X400 = Occurs when the system detects indications of sufficient growth in the growth control (GC) tubes in less than four days.  

 

YES

YES
How can you troubleshoot X200? Re-inoculation of the growth tube Repeat testing with a pure culture Subculture on Blood Agar Prepare a ZN smear 2

X 200 occurs due to insufficient growth within the specified protocol time (i.e., 13 days for  SIRE or 21 days for PZA) due to too little inoculum, nonviable organisms, or a slow-growing drug-resistant strain. In this case, repeat testing with a pure culture of the isolate is recommended.

   YES YES

 

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