Editors: Olga Pozdnyakova, MD, PhD, Geoffrey Wool, MD, PhD, David Bernard, MD, PhD & Raul S. Gonzalez, MD
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Q. Is a pathology review on all cerebrospinal fluid (CSF) specimen differential slides necessary? Should the review be based on the number of white blood cells counted or the abnormality of the differential, or both?
A. February 2025—Normal CSF is clear and colorless and has a WBC count of 0–5 WBC/μL in adults. It is the only sterile body fluid that has sufficient quantities for ease of sampling in healthy individuals. Most CSF collections are used to diagnose and guide treatment of conditions such as meningeal infection, subarachnoid hemorrhage, central nervous system involvement in malignancy, and demyelinating diseases. CSF specimens from patients who have meningitis or have had a traumatic tap yield the highest WBC counts.
Most body fluid differential slides are prepared using cytocentrifugation to concentrate the specimen into a smaller, more manageable counting area while maintaining cell integrity. Cytocentrifugation is known to distort the appearance of cells by making them appear stretched or by creating artifacts within the cells. Most CSF specimens have a low WBC count, so very few cells are seen on prepared slides. It is imperative that slides are scanned thoroughly to ensure all cells are reviewed.
Cytocentrifugation and low cell counts may contribute to pathology departments wanting to review each CSF specimen. Though distortion is possible, cells on a slide prepared via cytocentrifugation closely mimic cells seen in a peripheral blood smear. The former keep their nucleus to cytoplasm ratio and distinguishing features, making them readily identifiable in body fluid specimens. Malignant cells, blast and blast-equivalent cells, and plasma cells will not lose any features from being placed on a slide via cytocentrifugation. Therefore, laboratory personnel who are trained to perform initial smear reviews and have acceptable performance on the related proficiency testing can identify abnormalities in specimens and determine the need for pathology review, no matter the WBC count.
Keohane EM, Otto CN, Walenga JM. Rodak’s Hematology: Clinical Principles and Applications. 6th ed. Saunders; 2020.
Kjeldsberg C, Knight J. Body Fluids: Laboratory Examination of Amniotic, Cerebrospinal, Seminal, Serous & Synovial Fluids. 3rd ed. American Society of Clincial Pathologists; 1993.
Meagan Seeger, MLS(ASCP)CM
Hematology and Coagulation Supervisor
Wisconsin Diagnostic Laboratories, Milwaukee, Wis.
Member, CAP Hematology/Clinical Microscopy Committee
The following question and answer was first published in January 2021. Periodically we will republish answers to questions that remain important and current. At the time of initial publication, Dr. Chen was chair of the CAP Hemostasis and Thrombosis Committee and Dr. Ravindran was with the Mayo Clinic, Rochester, Minn., and member of the CAP Hemostasis and Thrombosis Committee.
Q. At what level or time is aPTT considered incorrect? Is an aPTT of less than 22.0 seconds an acceptable result?
A. Interpretation of activated partial thromboplastin time (aPTT) depends on the reference range, which is established in every laboratory in accordance with guidelines from the Clinical and Laboratory Standards Institute.1 The recommendation is for each laboratory to initially establish a reference range or verify the reference range of an FDA-approved test. This normal range should be verified with any change in reagent, lot number, instrument, or collection system, or once per year.2
A falsely prolonged aPTT is one of the most common outcomes of a clinical laboratory aPTT result generated incorrectly. False elevation can be a consequence of specimen collection and handling issues, such as failure to correct the anticoagulant volume in patients with a hematocrit greater than 55 percent, underfilling anticoagulant for a recommended collection volume of whole blood to anticoagulant ratio of 9:1, and contamination with heparin. Another cause is increased sensitivity of aPTT reagents (greater than 50 percent). The recommended factor sensitivity for aPTT reagents is within 30 to 45 percent.1
Furthermore, prolonged aPTT can be a consequence of factor deficiency—most commonly factors VIII, IX, XI, and XII with less than 30 percent activity or less than 0.3 U/mL—or the presence of a nonspecific inhibitor, such as lupus anticoagulant, or a specific coagulation factor inhibitor.
With regard to the second question, reference ranges tend to vary by laboratory, as laboratories may use different manufacturers’ aPTT reagents or kits. However, an aPTT of less than 22.0 seconds could be considered a shortened time if it is shorter than the lower reference interval of aPTT. The most efficient way to confirm a shortened aPTT is to collect another blood sample and repeat confirmation testing.3
Shortened aPTT can be attributed to preanalytical variables, disease conditions, and normal biological variability. Among the preanalytical variables are a suboptimal specimen due to a difficult or poor blood draw or a partially clotted sample, or inappropriate specimen collection and handling, such as overfilling a blood collection tube. A shortened aPTT can also be caused by a hypercoagulable state with increased predisposition to thrombosis, such as in postoperative patients or coagulation disorders such as factor V Leiden mutation and antithrombin deficiency. It is also common in patients with an acute or chronic condition—for example, myocardial infarction or malignancy—and inflammation. Finally, since a normal range is established by having approximately 2.5 percent of normal healthy people’s results outside either side of the cutoffs, a shortened aPTT could simply reflect individual biologic variability within that 2.5 percent range.4
- Clinical and Laboratory Standards Institute. H47-A2: One-Stage Prothrombin Time (PT) Test and Activated Partial Thromboplastin Time (APTT) Test; Approved Guideline, 2nd ed.; 2008.
- Castellone DD. Establishing reference intervals in the coagulation laboratory. Int J Lab Hematol. 2017;39(suppl 1):121–127.
- Lippi G, Salvagno GL, Ippolito L, Franchini M, Favaloro EJ. Shortened activated partial thromboplastin time: causes and management. Blood Coagul Fibrinolysis. 2010;21(5):459–463.
- Lippi G, Favaloro EJ. Activated partial thromboplastin time: new tricks for an old dogma. Semin Thromb Hemost. 2008;34(7):604–611.
Dong Chen, MD, PhD
Division of Hematopathology
Department of Laboratory Medicine and Pathology
Mayo Clinic, Rochester, Minn.
Aishwarya Ravindran, MD
Assistant Professor of Pathology
Division of Laboratory Medicine, Hematopathology
University of Alabama at Birmingham
Member, CAP Hematology/Clinical Microscopy Committee