Q & A, 08/13

University of Alberta Hospital

Edmonton, Alberta

Member, CAP Standards Committee
[pulledquote]Q. We are using the Advia Centaur assay to measure testosterone, which has a level of detection of 10 ng/dL. Will reporting <10 ng/dL be sufficient for clinicians to diagnose a child? [/pulledquote]

A. There are three aspects to this question: 1) What is the lowest value one can reliably report with any given assay? 2) How are testosterone measurements used in the care of children? 3) Are testosterone immunoassays in general (and Advia Centaur in particular) reliable enough to use in pediatric patients?

With respect to the first question, two terms are commonly used (and confused) in determining the lowest values one can report: limit of detection and limit of quantitation.¹ The first, limit of detection, is typically determined by performing the assay on the zero standard 20 times, and then determining from the standard curve the concentration corresponding to the mean plus two standard deviations of those measurements. The latter, limit of quantitation, is comparable to “functional sensitivity”; it is always a higher concentration than the limit of detection. The limit of quantitation is determined by finding the imprecision of pools of patient samples at various (low) concentrations and finding the lowest concentration at which the coefficient of variation does not exceed some arbitrary limit (usually 10 percent or 20 percent).

The package insert for the Advia Centaur testosterone assay2 cites the limit of detection as 10 ng/dL but provides no value for the limit of quantitation.  For comparison, another manufacturer’s package insert for its testosterone immunoassay cites a limit of detection of 2.50 ng/dL and a limit of quantitation, using a 20 percent threshold for imprecision, of 12.0 ng/dL.

To answer the second question, testosterone measurements in children are generally made for the classification and monitoring of congenital adrenal hyperplasia and adrenal insufficiency, in the diagnosis of polycystic ovarian syndrome in girls, and in evaluating precocious or delayed puberty in boys. It is only in evaluating delayed puberty that precise determination of results <10 ng/dL is likely to be important.

Perhaps more important, though, is the third question: Should immunoassays for testosterone be used at all for pediatric samples? Although the assays may be “FDA approved,” many experts believe that most immunoassays are not reliable enough to be used in pediatric (or even adult female) patients, because values in these populations are typically in the low range (1–50 ng/dL), where imprecision and cross-reacting substances may cause clinically significant problems.3-7

In other words, for Advia Centaur in particular, for which the manufacturer provides no limit of quantitation, the reproducibility of all results in the low range (even those with numbers above 10 ng/dL) may be poor, and there are published data to suggest that numerical values obtained may not represent testosterone at all.
More generally, testosterone immunoassays, regardless of manufacturer, should be used with caution, if at all, in children and in adult females.

1. Clinical and Laboratory Standards Institute. Protocols for determination of limits of detection and limits of quantitation; approved guideline. CLSI document EP17-A. Wayne, Pa.: CLSI;2004.
2. Siemens [package insert]. Testosterone 111751 Rev. N, 2009-07.
3. Rosner W, Auchus RJ, Azziz R, Sluss PM, Raff H. Position statement: Utility, limitations, and pitfalls in measuring testosterone: an Endocrine Society position statement. J Clin Endocrinol Metab. 2007;92(2):405–413.
4. Taieb J, Mathian B, Millot F, et al. Testosterone measured by 10 immunoassays and by isotope-dilution gas chromatography-mass spectrometry in sera from 116 men, women, and children. Clin Chem. 2003;49(8):1381–1395.
5. Herold DA, Fitzgerald RL. Immunoassays for testosterone in women: better than a guess? Clin Chem. 2003;49(8):1250–1251.
6. . Accessed Nov. 2, 2012.
7. Kushnir MM, Blamires T, Rockwood A, et al. Liquid chromatography-tandem mass spectrometry assay for androstenedione, dehydroepiandrosterone, and testosterone with pediatric and adult reference intervals. Clin Chem. 2010;56(7):1138–1147.

Gary L. Horowitz, MD
Medical Director, Clinical Chemistry
Beth Israel Deaconess Medical Center
Boston
Associate Professor
Harvard Medical School

Chair, CAP Chemistry Resource Committee
Daryl Erik Palmer-Toy, MD, PhD
Assistant Medical Director of Laboratory Services, Southern California
Permanente Medical Group
Medical Director, SCPMG Regional Reference Laboratories
North Hollywood

Member, CAP Chemistry Resource Committee

[pulledquote]Q. An age-old problem exists regarding the differentiation of allergic rhinitis/sinusitis from an infectious etiology (bacterial or viral). The distinction has been based primarily on clinical signs and symptoms. Would a simple nasal swab stained for eosinophils help differentiate an allergic from an infectious etiology?[/pulledquote]

A. Unfortunately, the presence of eosinophils or Charcot-Leyden crystals, or both, is not completely specific for allergic rhinitis/sinusitis. This is because allergic fungal sinusitis is an entity that includes the presence of viable fungal elements in an allergic mucous. Apart from this entity, the presence of eosinophils and evidence of their products is definitely useful, in conjunction with other studies, for the diagnosis of allergic rhinitis/sinusitis.

Chang C, Gershwin ME, Thompson GR 3rd. Fungal disease of the nose and sinuses: an updated overview. Curr Allergy Asthma Rep. 2013;13(2):152–161.

Gary W. Procop, MD, MS
Chair, Molecular Pathology
Section Head, Clinical and
Molecular Microbiology
Director, Parasitology and
Mycology Laboratories
Professor of Pathology
Cleveland Clinic

Chair, CAP Microbiology
Resource Committee
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Dr. Kiechle is medical director of clini­cal pathology, Memorial Healthcare, Hollywood, Fla. Use the reader service card to submit your inquiries, or address them to Sherrie Rice, CAP TODAY, 325 Wau­ke­gan Road, Northfield, IL 60093; srice@cap.org.