Q&A column

In the context of toxicology testing, an increased serum osmol gap (the difference between calculated and measured osmolality) may suggest the presence of otherwise unmeasured osmotically active substances, including volatile alcohols. While direct testing for the more common alcohols is generally readily available, the osmol gap may support a clinical suspicion for intoxication while more definitive laboratory workup is pending.

Measurement of serum, urine, and stool osmolality plays a key role in the diagnostic workup and monitoring of several clinical conditions. Although in some settings its utility and practicality are limited by the availability of alternative testing, such as direct ethanol testing in the setting of suspected intoxication or urine specific gravity to estimate renal concentrating ability, osmolality measurement remains the best test in certain clinical scenarios.

1. Rifai N. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, 6th ed. St. Louis, Mo.: Elsevier; 2018.
2. Goldman L, Schafer AI. Goldman-Cecil Medicine, 25th ed. Philadelphia: Elsevier Saunders; 2016.

Sammie Roberts, MD,
Department of Pathology
University of Colorado School of Medicine
Aurora, Colo.
Former member, CAP ClinicalChemistry Committee

Sridevi Devaraj, PhD, D(ABCC)
Professor, Pathology and Immunology
Baylor College of Medicine
Director, Clinical Chemistry and
Point of Care Testing
Associate Director, Microbiome Center
Texas Children’s Hospital, Houston
Member, CAP Clinical Chemistry Committee

Q. When a result is outside the analytical measurement range, the sample is processed by dilution. If the result of that dilution is found within the AMR, it is then multiplied by the dilution factor and reported. Should we include a comment on the report saying the result was obtained after dilution?

A. For results greater than or less than the analytical measurement range, a numeric result must not be reported unless the sample is processed by dilution, a mixing procedure, or concentration to obtain a value that falls within the AMR. The result must be within the AMR before it is mathematically corrected by the concentration or dilution factor to obtain a reportable numeric result. Although the laboratory is required to retain testing records, including mathematical corrections, there is no requirement to include a comment on the patient report indicating that a result was obtained after dilution.

1. Medicare, Medicaid, and CLIA programs; laboratory requirements relating to quality systems and certain personnel qualifications. Fed Regist. 2003;68(16):3712. To be codified at 42 CFR § 493.1282(b)(1)(ii).
2. College of American Pathologists. CHM. 13710 Diluted or concentrated samples. In: Chemistry and toxicology checklist. Aug. 22, 2018.

Joel Graff, MBA, MT(ASCP)
Senior Technical Specialist
Laboratory Accreditation Program
College of American Pathologists
Northfield, Ill.