The SAGE IVD recommended conditional inclusion on the EDL of the IVD for enumeration of lymphocyte subtypes for diagnosis of immunodeficiency, pending submission of evidence for its clinical usefulness and use in LMICs. The Group noted that the test is expensive and requires highly skilled laboratory technicians. No evidence was provided of its use in LMICs. Furthermore, the submission did not include an evaluation of evidence that patients were treated with the associated EML drugs as a result of use of the test, and no evidence was given for its diagnostic accuracy or safety. No guidelines for its use appear to be available. Recommended test purpose: as an aid in the diagnosis of primary and secondary immunodeficiencies. SAGE IVD reviewed additional evidence in 2020 (available at: https://iris.who.int/handle/10665/339064) and recommended reversing the conditional listing on the enumeration of lymphocyte subtypes (CD4, CD8, CD20 and CD16/56) test, therefore this test was fully listed in EDL 3.

Immunodeficiency consists of a wide variety of conditions and is underdiagnosed even in high-income countries. It contributes to a significant disease burden, which can often be relieved if the condition is diagnosed early and treated appropriately. While estimates of immunoglobulin are widely used, they do not identify the cellular-based abnormalities, and a combined diagnostic approach is required. Both specific treatment and long-term care of chronically infected patients are expensive, with wide implications for the families of sufferers. The capital costs of a flow cytometric facility are high, as a dedicated laboratory with access to a reliable power supply and consumables is required. A flow cytometry facility for following up HIV patients and/or for diagnosis of leukaemia can also be used for immunodeficiency testing. This IVD for enumeration of lymphocyte subtypes is used in protocols to ensure swift diagnosis according to the clinical presentation; however, the technique requires significant manpower, training and maintenance, a dedicated machine and expertise and training for interpretation. The test is used and is recommended by a number of professional associations. The 2020 resubmission contains more comprehensive (and more recent data) on the importance and utility of lymphocyte subtype analysis in diagnosing primary immunodeficiency disorders (PIDs). Immunoglobulin analysis is a good diagnostic aid for PID, but it only identifies antibody-deficient immune deficiencies. The additional data from lymphocyte studies helps categorize these deficiencies but also identifies the important group of cellular deficiencies. Studies by the applying group and others show that access to flow cytometry in the diagnosis of PIDs is patchy across LMICs (1, 8, 9). This is unsatisfactory. Flow cytometry is now well established for other conditions and widely available in many LMICs and could be expanded to include PID studies; this is probably already the case in many countries. It may not be costeffective to buy flow cytometers purely for PID studies (unless perhaps in a large specialist centre), but extending the use of equipment that is already available should be encouraged.

No detailed evidence or summary of evidence was submitted to support this intended use. There is evidence of the accuracy and value of flow cytometry in differentiating lymphocyte subsets, particularly for diagnosis of leukaemia, extended to HIV infection and lymphoma. There is also clear evidence of the importance of accurate, early diagnosis in reducing morbidity and mortality and of cost–effectiveness. As flow cytometry is a routine diagnostic technique, with immunoglobulin estimation in high-income settings, the test has not been investigated individually. Its importance has been accepted by association. SAGE IVD reviewed additional evidence in 2020 and recommended reversing the conditional listing on the enumeration of lymphocyte subtypes (CD4, CD8, CD20 and CD16/56) test. A summary of the additional evidence and the list of references are available at https://iris.who.int/handle/10665/339064

World Health Organization. (‎2019)‎. The selection and use of essential in vitro diagnostics: report of the second meeting of the WHO Strategic Advisory Group of Experts on In Vitro Diagnostics (‎including the second WHO model list of essential in vitro diagnostics)‎. World Health Organization. https://iris.who.int/handle/10665/329527 ; World Health Organization. (‎2021)‎. The selection and use of essential in vitro diagnostics: report of the third meeting of the WHO Strategic Advisory Group of Experts on In Vitro Diagnostics, 2020 (‎including the third WHO model list of essential in vitro diagnostics)‎. World Health Organization. https://iris.who.int/handle/10665/339064