Sir,

We thank Dr Kittisupamongkol1 for his interest in our case and for drawing attention to a key aspect of diagnosing primary immunodeficiency—serum antibody levels reflect rate of production and loss and this can be influenced by a long list of pathologies which must be excluded before diagnosing CVID. We can confirm that our case meets full diagnostic criteria as outlined by the European Society of Immunodeficiency,2 and that further investigation was undertaken to identify new pathologies such as mycobacterial infection.

Where immune deficiency is suspected, an initial panel of investigations should be undertaken to exclude haematological malignancy autoimmune disease, renal disease, and HIV. A thorough medical, surgical, and drug history should identify iatrogenic immune deficiency. Further investigations to exclude defined cellular immunodeficiency or genetic disorders can be undertaken by the immunology laboratory.3

Conflict of interest

The authors declare no conflict of interest.