Editor: Frederick L. Kiechle, MD, PhD
[printfriendly]
Submit your pathology-related question for reply by appropriate medical consultants. CAP TODAY will make every effort to answer all relevant questions. However, those questions that are not of general interest may not receive a reply. For your question to be considered, you must include your name and address; this information will be omitted if your question is published in CAP TODAY.
[button size=”small” link=”http://www.captodayonline.com/q-a-submission/”]Submit a Question[/button]
[hr]
[pulledquote]Q. We are standardizing procedures across our system. In thrombocytopenic patients with low platelet counts, some sites perform manual platelet counts using the Unopette system; others perform slide estimates to confirm an automated count. The need for improved turnaround times and greater accuracy and precision is clear. Are there studies that have evaluated the true accuracy of a low platelet count via a manual dilution technique versus the many automated techniques, and is there a true value in performing the time-consuming manual platelet count?[/pulledquote]
[dropcap]A.[/dropcap] In our experience, manual platelet counts are generally not recommended in the context of thrombocytopenia detected by automated complete blood count. Manual phase contrast microscopy is time-consuming and imprecise at low platelet counts.(1) Pathologists and treating clinicians should also be aware of the significant limitations, with respect to accuracy and precision, of automated cell counters in patient samples with severe thrombocytopenia (less than 20×109 platelets/L).(2) We would also note that clinical correlation and peripheral smear review are further suggested as optimal clinical practice to exclude pseudo-thrombocytopenia due to artefact (for example, platelet clumping, platelet satellitism due to EDTA), as well as an alternative approach to the correct interpretation of automated cell counters reporting results outside of the verified reportable ranges.
In 2001, in the absence of a true reference method for direct enumeration of platelets, the International Council for Standards in Haematology (ICSH) issued a recommendation for an indirect platelet count using immunologically labeled platelets detected by a flow cytometer relative to an RBC count determined with a semiautomated, single-channel, aperture-impedance particle counter.3 Through interlaboratory studies, this ratiometric method was shown to be sufficiently accurate and precise to be used for whole blood calibration of cell counters and for assigning values to calibration materials, which was the intent of the ICSH guideline.(4) This is the current recommendation for any validation of low platelet counts to which all hematology blood counter manufacturers are held as the international standardization protocol. However, this ICSH method is neither intended nor practical for platelet count verification in the clinical setting. The ICSH method could be used as part of a labora-tory-developed test if a clinical laboratory is seeking to extend the linearity claim beyond that provided and calibrated by the instrument manufacturer.
A further suggestion outside the realm of technical advice is to discuss the clinical implications of your low platelet count reporting SOP for your laboratory with both the laboratory director and the appropriate ordering or treating physicians. In particular, know what the accepted platelet count trigger is for platelet transfusion. Often the laboratory spends valuable time worrying about and verifying what actual platelet count number should be reported, when the clinically relevant information desired by treating physicians is as simple as “is the count above or below 10,000/μL? 20,000/μL? 50,000/μL?” In practical terms, often the best practice is a quick smear review by an experienced morphologist who is empowered to accept or override the automated blood cell counter reported result.
- De la Salle BJ, McTaggart PN, Briggs C, et al. The accuracy of platelet counting in thrombocytopenic blood samples distributed by the UK National External Quality Assessment Scheme for General Haematology. Am J Clin Pathol. 2012; 137(1):65–74.
- Trabuio E, Valverde S, Antico F, Manoni F, Gessoni G. Performance of automated platelet quantification using different analysers in comparison with an immunological reference method in thrombocytopenic patients. Blood Transfus. 2009;7(1):43–48.
- Platelet counting by the RBC/platelet ratio method. A reference method. International Council for Standardization in Haematology Expert Panel on Cytometry; International Society of Laboratory Hematology Task Force on Platelet Counting. Am J Clin Pathol. 2001;115(3):460–464.
- Harrison P, Ault KA, Chapman S, et al. An interlaboratory study of a candidate reference method for platelet counting. Am J Clin Pathol. 2001;115(3):448–459.
Jay L. Patel, MD, Clinical Assistant Professor
University of Calgary, Calgary, Alberta
Member, CAP Hematology and Clinical Microscopy Resource Committee
Bruce H. Davis, MD, President and Founder
Trillium Diagnostics, LLC, Bangor, Me.
Former member, CAP Hematology and Clinical Microscopy Resource Committee
[pulledquote]Q. Our laboratory is receiving requests for urinary eosin-ophils from a new internist who insists this is the gold standard test for diagnosing acute interstitial nephritis. My understanding is that although it is a relatively easy test to perform, the test lacks sensitivity and specificity. What can you tell me about it?[/pulledquote]
[dropcap]A.[/dropcap] Published studies on the clinical utility of urinary eosinophils for diagnosing acute interstitial nephritis (AIN) are limited, with most reported in the 1980s and 1990s. These studies show conflicting data and suffer from a variety of shortcomings, including lack of comparison with the gold standard, renal biopsy; differences in testing methodology (preparation, staining, and quantification of urine eosinophils); and variable use of alternative criteria for establishing the diagnosis of AIN when renal biopsies were not performed. Initial enthusiasm suggested that the finding was relatively specific for AIN.(1)
The most common method for detecting urinary eosinophils is based on microscopically enumerating the frequency of eosinophils in air-dried, stained urinary sediment samples, with normal urine containing less than one percent eosinophils. A positive test is defined as greater than one percent or greater than five percent eosinophils, with the high number being more specific for AIN. Hansel stain (methylene blue and eosin Y) is most useful for distinguishing eosinophils from neutrophils in urine and other body fluid. Wright and Giemsa stains perform poorly in low pH samples. In 11 patients with interstitial nephritis, eosinophils were detected in 10 using Hansel stain but in only two using Wright stain.(2) One report suggested that even though Hansel stain was easier to interpret, deficiencies in Wright-stained samples could be overcome by more experienced observers who found no difference in eosinophil frequency between the two methods.(3) Hansel reagents and staining methods are commercially available and result in eosinophil granules staining bright red and easily distinguishable from blue neutrophils.
Subsequent to this initial enthusiasm, reports appeared that showed that eosinophils can be increased in the urine in a variety of other urinary tract disorders, among them cystitis, prostatitis, pyelonephritis, glomerulonephritis, and atherosclerotic embolic renal disease. Using the most common definition of eosinophiluria (greater than one percent eosinophils), false-positive rates of four percent were found in unselected patients with pyuria. In the same study, a positive urine eosinophil test result occurred in only 40 percent of patients diagnosed with AIN by a nephrologist who based the diagnosis on clinical grounds and a limited number of renal biopsies. Ten of 36 patients with renal disease other than AIN had a positive test indicating that the test was neither specific nor sensitive, with positive predictive value no better than that determined by the flip of a coin.(3) Current status on the usefulness of the test is well summarized in a letter to the editor of the New England Journal of Medicine in 2008.(4) For now, use and interpretation of the presence and quantities of eosinophils in urine sediment must be interpreted in the context of patient-specific features and should not be over interpreted as indicating AIN or any other specific renal condition. Nor should the absence of eosinophils necessarily exclude the diagnosis of AIN.
- Corwin HL, Korbet SM, Schwartz MM. Clinical correlates of eosinophiluria. Arch Intern Med. 1985;145:1097–1099.
- Nolan CR III, Anger MS, Kelleher SP. Eosinophiluria—a new method of detection and definition of the clinical spectrum. N Engl J Med. 1986;315:1516.
- Ruffing KA, Hoppes P, Blend D, Cugino A, Jarjoura D, Whittier FC. Eosinophils in urine revisited. Clin Nephrol. 1994;41:163.
- Fletcher A. Eosinophiluria and acute interstitial nephritis. N Engl J Med. 2008; 358:1760–1761.
David L. Zwick, MD, Director of Hematology Laboratory,
Department of Pathology and Laboratory Medicine,
The Children’s Mercy Hospital
Kansas City, Mo.
Member, CAP Hematology and Clinical Microscopy Resource Committee