• Title: MELISA test control values
• Content text:

  The implication of positive and negative controls in the MELISA test

  
  To ensure the accuracy and reliability of MELISA testing, both positive and negative controls must be performed alongside antigen testing. These controls confirm that the test has functioned correctly and that lymphocytes respond appropriately and specifically to the tested antigens.

  Positive Control – Pokeweed Mitogen (PWM)

  The positive control uses Pokeweed Mitogen (PWM), a substance that should trigger a strong immune response in all individuals. This step verifies whether the patient's immune system is capable of mounting a cellular reaction.

  If the cells fail to react properly to the positive control, it suggests that the immune system is not functioning optimally or that cells are weakened. Possible reasons for this include:

  • Immunosuppressant medication, which may dampen the immune response.
  • Cell viability issues after sample collection - weakened or damaged cells (e.g., due to prolonged transport, transport conditions, incorrect temperature range).

  For the test to be considered valid, the reaction to PWM must be not just present but also very strong. A weak response to PWM indicates reduced test sensitivity, making it harder to detect weaker immune reactions to specific antigens.

  Negative Control – Baseline Cell Activity

  The negative control measures the baseline activity of the immune cells, with expected values typically ranging between 500 and 3,000 cpm. A low negative control ensures a clear distinction between normal background activity and responses to the antigens being tested.

  If the negative control values are too high, the overall test sensitivity decreases, making it harder to detect weak/slight responses and/or to differentiate weak, moderate, and strong specific reactions to antigens.

  An elevated negative control suggests that the immune system was already stimulated at the time of blood collection, which can be caused by:

  • Recent or ongoing viral/bacterial infections
  • Recent vaccination
  • Exposure to mold
  • Organ transplantation

  Stimulation Index (SI) – Evaluating Immune Response

  The Stimulation Index (SI) of PWM must be at least 100 for the test results to be fully interpretable.

  • SI between 50-100: Indicates reduced test sensitivity, making it harder to distinguish subtle immune responses to antigens. In such cases, the morphology analysis becomes even more essential for interpretation.
  • SI below 50: Results are generally unreliable and cannot be evaluated.

  By ensuring appropriate positive and negative control values, the MELISA test maintains high sensitivity, allowing for the detection of even weak immune responses to tested antigens.

   

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