Clinical Predictors of Abnormal Peripheral Blood Lymphocytoses Diagnosed by Flow Cytometry: An Algorithmic Approach That Can Be Applied in Routine Clinical Practice.
Authors: Andrews Jared M. , Holm Mitchel T. , Myers Jerome B. 
Published: April 2020

Abstract
Background
Elevated peripheral blood lymphocyte counts in adults can occur in benign reactive conditions as well as malignant disease processes. Chronic lymphocytic leukemia (CLL) is the most common adult hematologic malignancy of the western world affecting the middle aged and elderly. Less commonly B, T, and Natural Killer (NK) cell leukemia / lymphomas may also present with lymphocytosis. Flow cytometry has greatly improved the ability to detect low levels of abnormal lymphocyte populations in peripheral blood. It is, however, a relatively expensive test and clinical guidelines for its appropriate usage are not well defined.
Methods
We conducted a retrospective review of peripheral blood lymphocytoses that were submitted for flow cytometric analysis at Madigan Army Medical Center, Tacoma, WA from 2002 – 2004. Under laboratory protocol, all patients ≥ 50 years of age with an absolute lymphocyte count (ALC) of &gt; 4 X 109 Cells/L had a peripheral smear evaluated by both a hematology technician and pathologist. Specimens determined to warrant flow cytometric analysis based on review of clinical history, prior lab values, degree of lymphocytosis, and morphology were either recommended for flow cytometry in a comment; or sent directly for analysis with the clinician’s approval. We reviewed complete blood counts (CBCs), previous flow cytometry results, as well as bone marrow and electronic clinical history. All patients with previous diagnoses of lymphoproliferative disorders (LPDs) or ALC &lt; 4 X 109 Cells/L were excluded.
Results
Approximately 7,300 CBC specimens/month (3,400 from patients ≥ 50 years of age) were performed. Of these, an average of 44 specimens/month had a lymphocytosis of ≥ 4 X 109 Cells/L, from approximately 28 different patients. From this group 71 flow cytometric cases (an average of 2/month) were performed over the 2 year period. 42 cases (59%) had an abnormal phenotype. 27 had a phenotype consistent with CLL, and the other 15 were a mixture of LPDs involving B and T-lymphocytes as well as NK cells. Comparing normal phenotype to abnormal phenotype showed statistically significant differences between the mean age (n-60.4 ±7.5, abn-69.8±8.7), ALC (n-4.9±0.8, abn-9.2±8.1), and relative lymphocyte count (RLC) (n-43.9±7.5%, abn-59.3±8.8%).
Conclusion
Absolute lymphocyte counts ≥ 4 X 109 Cells/L in adults ≥ 50 years of age represent approximately 1% of the CBCs performed in our laboratory. Review of these cases by a pathologist is logistically feasible due to the low incidence. Our method of reviewing for morphology, clinical history, and past lymphocyte counts with comments to the ordering clinician yielded a high incidence of abnormal phenotype diagnoses when evaluated by flow cytometric analysis (59%). Age, ALC, and relative lymphocyte counts are variables that can be used to develop guidelines for determining the appropriateness of flow cytometric analysis. Patients &lt; 52.4 years of age fall below two standards of deviation from the mean age of the abnormal phenotype group. The standard of deviation for mean ALC is very small (4.9±0.8), which indicates that counts &gt; two standards of deviation above the mean, or 6.5 X 109 Cells/L, would correlate strongly with an abnormal phenotype. The same conclusion could be made with a RLC &gt; 58.9%. In conclusion, patients ≥ 50 years of age with an ALC &gt; 6.5 X 109 Cells/L or a RLC &gt; 58.9% are likely to have a lymphoproliferative disorder and flow cytometric analysis is indicated.