Standardized reagents and guidelines for the immunophenotyping of hematologic neoplasms by flow cytometry

Dr. Lisa Holey

Sandra Hernandez

This page will

  • Introduce the benefits of flow cytometric immunophenotyping for hematologic neoplasms
  • Provide an overview of the consensus guidelines and regulations governing flow cytometry for clinical use
  • Consider the benefits of standardized reagents and operating procedures over laboratory developed tests


Cancer incidence and mortality are growing rapidly worldwide, attributed to both aging and population growth, as well as changes in the prevalence and distribution of major risk factors.1 Hematologic malignancies (blood cancers) account for approximately 6% of all cancer cases, with close to 1.2 million new diagnoses registered in 2020, and nearly 700,000 deaths.1 The growing burden of blood cancers makes it increasingly important to effectively diagnose and characterize the disease early, to allow the best selection of treatment as soon as possible.2

Hematopoietic malignancies are diseases of immune system cells and are traditionally categorized according to where the cancer is first detected: in the blood (leukemias); lymph nodes (Hodgkin and non-Hodgkin lymphomas); or bone (myelomas). According to 2020 figures, non-Hodgkin lymphoma is the most common type of blood cancer, followed closely by leukemia.1 Leukemias are derived from changes to blood cells that can occur at any time during hematopoiesis, and lymphomas result from malignant transformations in lymphocytes. The exact point at which these transformations occur gives rise to the specific types of disease, and therefore lineage identification is exceedingly important in classifying the exact condition. There are several subtypes of blood cancers, which are based on surface markers, tumor architecture, cell morphology and differentiation, as well as genetic alterations. Early identification of the correct subtype is important to optimize treatment.3

Flow cytometric immunophenotyping

Flow cytometric immunophenotyping (FCI) is a useful tool for the diagnosis, classification, staging, and monitoring of hematologic malignancies. The technique relies on a panel of antibodies that detect markers on the cells, aiding the identification of cancerous cells and their lineage. The ability to show subtle differences in antigen density also allows this technique to distinguish and characterize aberrant cells that may be present in tiny quantities.4 Flow cytometry has several advantages over immunohistochemistry (IHC) – the method that has traditionally been used to diagnose blood cancers – and these include5:

  • Defining distinct cell populations by their size and granularity
  • Identifying dead cells to remove them from the analysis
  • Detecting weakly expressed surface antigens
  • Measuring several intracellular or surface antigens simultaneously, using multicolor analysis
  • Delivering results quickly

Until recently, there were no commercially available in vitro diagnostic (IVD) panels for carrying out FCI for hematologic malignancies, and so clinical diagnostic centers have depended on Laboratory Developed Tests (LDTs). However, whichever methodology is used, it is of high importance that laboratories consult the available classifications and consensus guidelines for blood cancers, as well as any local regulations for IVD tests, to ensure that patients receive an accurate diagnosis that will guide their future treatment pathway. An overview of these guidelines and regulations is presented in this white paper.

For many years, the diagnosis of blood cancers was based on pathological and cytological examination of bone marrow and peripheral blood smears. However, as genetic research improved, it became clear that this technique did not reflect the full genetic and clinical diversity of any disease.6 In 2008, the 4th Edition of the ”WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues” – was published to help address this concern. One of the blue book monographs was published by the International Agency for Research on Cancer (IARC; Lyon, France), the classification aimed to recognize and classify different subgroups of hematopoietic neoplasms based on morphology, immunophenotyping, genomics, and clinical features. An important further revision to the 4th Edition was issued in 2017 as a result of emerging research with new clinical and biological implications.7, 8, 9 This update was created in collaboration with the Society for Hematopathology and the European Association for Haematopathology, and helped to build an international consensus among pathologists, hematologists, radiologists, and clinical oncologists. In addition to defining the malignancies, the WHO document also includes information on each tumor’s epidemiology, risk factors, and prognosis, as well as its ICD-O classification.10

The WHO continually updates its tumor classification references in order to represent the most up-to-date information and provide a uniform nomenclature of human cancers, which is accepted and used worldwide. A 5th Edition is anticipated soon.11

In 2006, the Bethesda Consensus conference identified significant variability both in the reagents used and in the formatting of results for FCI, despite the fact that the method had become common practice for the evaluation and monitoring of patients with hematopoietic neoplasias. Both elements are clearly important to allow results to be compared between different laboratories, and subsequently a committee of experts was formed to attempt to define a set of standards. The group included laboratory professionals from private, public, and university hospitals, as well as large reference laboratories, who routinely operated clinical flow cytometry for leukemia and lymphoma immunophenotyping. Together, they identified and published information on which cell lineages to evaluate for each variety of specific medical indications, and outlined a set of consensus markers suitable for the initial evaluation of each one.12 However, the variability of commercially available antibody clones, fluorescent tags, and cytometers, as well as the lack of clinical flow cytometers at the time that could assess more than five or six markers simultaneously, meant that the Bethesda participants could not agree on a single consensus standardized panel.

Nevertheless, this work began to define a wider set of consensus markers suitable for the initial evaluation of hematopoietic neoplasias, and it has since formed the basis for the development of both LDTs and commercial antibody panels.

Table 1. Consensus Reagents for Initial Evaluation for Hematopoietic Neoplasia12

Table 2. Reagents for Secondary Evaluation of Specific Hematopoietic Cell Lineages 12

With the more recent availability of 10-color+ flow cytometers, the barrier to limited panel sizes has largely disappeared, offering diagnostic labs using FCI the opportunity to provide wider screening panels, based on the consensus reagents described in the Bethesda guidelines.

CAP publishes instructions for leukemia and lymphoma flow cytometry proficiency testing and protocols for examining specimens from patients with suspected leukemias and non-Hodgkin lymphomas, including recommendations on markers to be assessed by immunophenotyping.13 Alongside this, the FDA has oversight of all IVD products in the US through its IVD Regulation.14 However, LDTs – such as those described above for FCI – can enter the market without FDA approval, or any other independent regulatory review, if they are created and used in the same facility. The Centers for Medicare & Medicaid Services (CMS) regulates clinical laboratories under the Clinical Laboratory Improvement Amendments (CLIA), but has limited insight into the quality, reliability, or usefulness of LDTs.15

Historically, the dependence on LDTs for FCI of hematologic malignancies has been a major area for dialog within the industry and, in 2009, CAP recommended further discussions concerning FDA oversight of these methods. This led, in March 2011, to the creation of a working group made up of international experts from the International Council for Standardization in Haematology (ICSH) and the International Clinical Cytometry Society (ICCS), aimed to address the validation of cell-based fluorescence assays specifically for clinical laboratories. The recommendations from this group were published in 2013 in a clinical cytometry special issue,16 and were submitted to the FDA for consideration as an official guidance document, and to the Clinical Laboratory Standards Institute (CLSI) as the basis for updated flow cytometry guidelines.

The FDA then convened a Public Workshop on ”Clinical Flow Cytometry in Hematologic Malignancies” to seek input from academia, government, laboratorians, industry, clinicians, patients, and other stakeholders on the role of clinical flow cytometry in hematologic malignancies, and to develop a specific regulatory policy for this class of in vitro diagnostic devices.17 The workshop discussed several issues including the increasing complexity of the LDTs in use, many of which used eight- and 10-color flow cytometry (as opposed to the three- and four-color platforms in use when the FDA first established guidance). These concerns were summarized in its October 2014 ”Draft Guidance for Industry, Food and Drug Administration Staff, and Clinical Laboratories: Framework for Regulatory Oversight of Laboratory Developed Tests (LDTs).”18 This was followed in January 2017 with the ”Discussion Paper on Laboratory Developed Tests (LDTs),”19 which noted that the organization would not be issuing a final guidance on the oversight of LDTs at that time.20

These initiatives and conclusions led the FDA to engage directly with flow cytometry vendors about future developments, and the first agency-authorized test for use with flow cytometry to aid in the detection of leukemias and lymphomas – ClearLLab Reagents (T1, T2, B1, B2, M) – was approved on June 29, 2017.21 In 2019 the ClearLLab 10C system, comprising four 10-color in vitro diagnostic panels of immunophenotyping reagents was cleared by the FDA for both lymphoid and myeloid lineages.22

More recently, in July 2021, ”Legislation for the Verifying Accurate Leading-Edge In Vitro Clinical Tests Development Act of 2020 (VALID Act)“23 was reintroduced to Congress, proposing that in vitro clinical tests should be separated from the existing medical device regulations, and that a regulatory framework for LDTs and IVDs should be established through a new FDA Center. This suggested legislation would require laboratories to comply with new requirements for registration with the FDA and, depending on the risk classification of their LDT, they might also have quality requirements, premarket review and approval, and adverse event reporting.24

The imminent change in regulations for IVD devices will have a significant impact on current FCI practice. Regulation 2017/746 on In Vitro Diagnostic Devices (IVDR)25 was agreed in 2017 between the European Council, the European Parliament, and the European Commission and, after a five-year transition period, it will replace the previous IVD Directive 98/79/EC on 26 May 2022. The new IVDR is a significant development and aims to strengthen the existing regulatory system in Europe, which has been in place for over 25 years. It includes some significant changes for both manufacturers of IVDs and clinical diagnostic labs running LDTs, including:

  • Classification system – IVDs will now be grouped into four different classes based on risk – from A (low) to D (high) – with regulation and assessment dependent on the class of device. Details of the classifications are included in Appendix VIII of the regulation.
  • Conformity assessment procedures – under the old directive, many IVDs could be self-certified and placed on the EU market solely under the responsibility of their manufacturers, without approval from any notified bodies. IVDs will now be subject to conformity assessment based on the classification of the device. Classes B, C, and D IVDs will all require assessment and certification by an appropriate notified body.
  • Performance evaluation and clinical data – the new regulations require a far more detailed performance evaluation of IVDs. Specific requirements are also defined in relation to the use of clinical data and clinical performance studies.
  • In-house manufacturing of IVDs – IVDs manufactured within a healthcare institution for its sole use were previously exempt from the regulations. The new IVDR applies to in-house IVDs.

Figure 1. LDT vs IVD validation workflow under the IVD Directive 98/79/EC and new IVDR regulations.

The new IVDR states that “devices intended to be used in screening, diagnosis or staging of cancer” – for example, flow cytometers – will be classified as class C devices (Annex VIII), meaning that they are subject to conformity assessment by a notified body and must meet the performance evaluation requirements.26 Clinical laboratories adhering to the new regulations will no longer have the option of using an LDT if a comparable certified IVD is commercially available. Labs that continue to use LDTs will be required to CE mark them and meet several new standards, including compliance with the IVDR’s Annex 1 “General Safety and Performance Requirements“ and quality management system framework. Even commercial clinical laboratories not based in the EU but testing specimens from European citizens with LDTs are likely to be considered “distance sales” and will also require CE IVD marking.27

The EuroFlow Consortium was set up in 2006, supported by the European Commission, as a Specific Targeted Research Project (STREP) of the 6th framework program. The project originally focused on developing flow cytometry for fast and sensitive diagnosis and follow-up of hematologic malignancies.28 Major goals included innovation – the development of new flow cytometry tools and strategies for the diagnosis and classification of hematologic malignancies – and reliability and reproducibility – assay standardization and automation across individual laboratories and countries.

The results of this project were published in 2012,29, 30 providing details on the selection of the most appropriate combination of fluorochromes for 8-color antibody panels, the protocols recommended for instrument settings, fluorochrome compensation and sample preparation, and the data analysis strategies adopted. The report also included the results of a multicentric evaluation of the reproducibility of these methods.

The standardized panels and SOPs developed by EuroFlow have since been validated across a wide range of ≥ 8-color flow cytometers from different vendors.31 For a significant period of time, these were the only antibody panels that had undergone a full technical and clinical multi-center validation and, for that reason, they were evaluated by many diagnostic centers worldwide against their local panels and immunophenotyping strategies. While many diagnostic labs have fully adopted the guidelines, others have adapted them to their settings and this, together with the freedom to buy reagents from any vendor, has contributed to inconsistency across different settings.

The EuroFlow consortium was incorporated into the European Scientific Foundation for Laboratory Hemato-Oncology at the end of its formal STREP project duration. Since then, it has continued to expand and continued its activities in the field of diagnosis and classification of leukemia and lymphoma.32

The advantages of standardized reagents for FCI

FCI has not yet fulfilled its potential to fully support the clinical diagnosis of hematologic malignancies. The reliance on LDTs still provides challenges to labs due to their time-consuming and complex set-up, and potential for inconsistent results, especially if SOPs are not strictly adhered to. The standardization of reagents and guidelines for FCI offers many benefits to both clinical laboratories and the clinicians and patients they serve.

Improved reliability

LDTs for FCI rely on the manual preparation of the antibody cocktail used in the screening panel. Any process involving manual pipetting is liable to errors, for example, because of mistakes made by the operator, or incorrect calibration of the pipettes. Furthermore, the use of wet reagents means that there is a risk of them degrading over time. Standardized, dry, pre-mixed antibody panels eliminate the risk of incorrect preparation. In addition, by adopting a standardized test with built-in controls, labs can be confident in the results produced.

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Reduced costs

There is a high capital cost associated with the purchase of a flow cytometer, so ensuring the efficiency and accuracy of the tests being run provides maximum return on this investment. The complexity of antibody preparation for LDTs can have significant cost implications as, in some labs, technicians are dedicated entirely to preparing the flow cytometry panels. Standardized reagents, however, can reduce manual input, waste, and errors, and can lead to considerable savings.

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Improved workflow efficiency

Often times, LDTs use manually prepared and designed panels, where each individual antibody must be prepared and then validated. Once this is complete, the cocktail must be prepared and then the antibodies revalidated in this combination. Further adjustments may also be required if the mixing of antibodies affects the validation. Rigorous documentation of compliance with local regulations must be maintained throughout for quality assurance, along with regular batch and stability testing. This represents a significant proportion of a workflow, especially when as many as 10 colors are being used in the most modern flow cytometers. Using standardized reagents not only helps to shorten workflows and enable faster results, but it also frees up technicians to focus on data analysis rather than repetitive manual tasks.

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Comparison across centers

Until recently, clinical data sharing and education have been hindered by the complexity and variability of test set-up and results analysis. Although different laboratories may base their tests on the same consensus guidelines, the lack of a single standardized approach means that results are often not comparable between institutions. Implementing standardized reagents and procedures across the global arena would provide a framework for cooperation, data sharing and education, improving the quality of both clinical education and research. Standardized approaches ensure that results conform with international guidelines, removing confounding factors such as variability in the set-up of protocols, and the interpretation of results.

Finally it is about confidence in results - for you and for your patients. Be Accurate

Support for clinicians and patients

Clinicians rely on the accuracy of results to make correct diagnoses and treatment decisions. Misinterpretation of samples and an incorrect clinical conclusion could have serious consequences for the patient. Standardized testing can strengthen confidence of clinicians in the results they are given, and the knowledge that the same conclusion would be reached wherever the analysis had been carried out, allowing them to provide a high quality of patient care. For patients, quicker and more accurate laboratory testing offers them the benefit of starting on the correct treatment pathway as soon as possible, increasing the chance of a good outcome.

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The adoption of standardized reagents and operating procedures in clinical diagnostic labs working on hematologic malignancies offers significant opportunities to improve efficiency and reliability, and reduce costs. This in turn benefits both clinicians and patients by ensuring treatment is started quickly and correctly. These tests offer the opportunity to be further developed to work with therapeutic markers to evaluate treatment efficacy, evaluate responders and non-responders, and identify minimal residual disease (MRD).

By ensuring that tests are consistent with the WHO consensus guidelines, the reported results will be recognized worldwide. Furthermore, by using an IVD that has met local regulations, labs will not be required to submit their LDTs for further evaluation in an area that is becoming increasingly regulated to ensure patient safety.


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