Designing a rational approach for the assessment of peripheral blood lymphocytosis in the acute care setting

Lymphocytosis is a common criterion used in hematology laboratories to refer a peripheral blood smear for pathologist review. Most centers employ a threshold absolute lymphocyte count above 5.0 × 109/L, but this approach lacks evidence and has poor specificity, especially in the acute care setting, where reactive causes for lymphocytosis are common. We sought to adjust the existing criteria to enhance laboratory efficiency, reduce the number of smears requiring pathologist review, and contribute to resource savings and improved patient care.

We retrospectively reviewed all consecutive cases of lymphocytosis over a 1-year period at a single center that met criteria for pathologist review. Differences in age, sex, and hematologic parameters were compared between patients with a neoplastic vs reactive cause for their lymphocytosis to establish new criteria for referral for pathology review. The impact of implementing these novel referral criteria on the volume of smears referred to pathology as well as their sensitivity and specificity for detection of previously undiagnosed lymphoproliferative disorders were assessed in the original patient cohort and in a validation cohort.

In 436 identified cases, pathologists noted concern for a previously unknown lymphoproliferative disorder in just 13 instances. Patients with a lymphoproliferative disorder had a higher absolute lymphocyte count and a lower neutrophil to lymphocyte ratio. Applying novel referral criteria based on patient age and neutrophil to lymphocyte ratio, we determined that the number of cases that required pathologist review could be reduced by more than 60% without compromising identification of true cases of -lymphoproliferative disorder.

Rational design of criteria for pathologist review of lymphocytosis has the potential to greatly reduce workload for pathologists and laboratory staff without -substantially affecting detection of true cases of lymphoproliferative disorder in the acute care setting.