Pancytopenia is a simultaneous deficiency of three blood cell lineages: red blood cells, platelets, and neutrophils. Its clinical significance is the triple impact of anemia (decreased tissue oxygen supply), thrombocytopenia (bleeding), and neutropenia (susceptibility to infection).

Confusion arises because the literature sometimes defines pancytopenia as low counts of red blood cells, platelets, and white blood cells.

Which is correct?

It helps to understand that the population of white blood cells actually contains at least four subtypes, depending on how you count: neutrophils (including eosinophils and basophils), monocytes, B- and T-lymphocytes.

• Neutrophils form a front-line defense against invading pathogens and respond to infections and injuries. They make up 40-70% of all peripheral white blood cells and are part of the innate immune system.
• Monocytes, also part of the innate immune system, are converted to tissue macrophages that engulf foreign substances.
• B lymphocytes participate in the humoral component of the adaptive immune system and secrete antibodies.
• T lymphocytes, so named because they develop in the thymus, are responsible for “cellular” immunity.

The monocytes, B and T cells are usually not reduced in number by conditions that cause pancytopenia, but neutrophils are. Out of the entire population of white blood cells, therefore, neutrophil count is the most relevant to the consequences of pancytopenia.

In our CDI Pocket Guide®, we have previously written that one of the diagnostic criteria for neutropenia is an absolute neutrophil count (ANC) of <1.8K.  Several readers have recently asked us whether a total white blood cell count (WBC) below normal is sufficient.

From a practical point of view, given that up to 70% of white cells are neutrophils, there is usually a correlation between a drop in WBC count overall and a drop in neutrophil number.

To really answer the question properly, however, it’s necessary to know how the ANC is calculated:

ANC = WBC x [percent neutrophils (also known as polymorphonuclear cells or PMN) + bands].

To illustrate the interdependence between WBC, ANC, and the diagnostic criteria (WBC <4k vs. ANC <1.8k), consider the following examples:

(1) Borderline WBC, borderline ANC, normal bands yields a borderline result:

            WBC 4,000 / PMN 40% / bands 5% = 4,000 x .45 = ANC of 1800

(2) Low WBC, borderline ANC, normal bands yields neutropenia and leukopenia

            WBC 3,800 / PMN 40% / bands 5% = 3,800 x .45 = ANC of 1710

(3) Lower WBC, normal PMN (range is 40-70%), and normal bands yields neutropenia and leukopenia:

            WBC 1,900 / PMN 60% / bands 10% = 1,900 x .70 = ANC 1,330

(4) Normal WBC, low neutrophils, no bands yields neutropenia without leukopenia:

            WBC 4,600 / PMN 35% / no bands = 4,600 x .35 = ANC of 1610

(5) Lowish WBC, normal PMN, no bands yields leukopenia without neutropenia

            WBC 3,500 / PMN 60% / no bands = 3,500 x .60 = ANC of 2100

As you can see from the above, a low WBC is a proxy for ANC, but it is not definitive.  The ANC really should be obtained to determine the presence of neutropenia.

Conclusion: Pancytopenia should be understood to be low RBC, low platelets, and low neutrophils.

We hope this clarifies this admittedly confusing matter, and welcome further comments and questions.

Get our CDI Pocket Guide® for more help with pancytopenia.