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Clinical laboratories are frequently asked to perform testing on nonstandard body fluids (e.g., peritoneal fluid, pleural fluid, synovial fluid, etc.), each with unique biochemical compositions. In healthy individuals, these fluids support mobility, viscoelasticity, and the local physiologic environment. In disease states, however, both the volume and composition of body fluids can change, making them valuable sources of diagnostic information. When these specimen types are not validated by the manufacturer, it is left to clinical laboratories to evaluate the performance of routine assays in these fluids.
Most routine chemistry assays are developed and validated for in vitro diagnostic use in common specimen types, such as plasma, serum, urine, or cerebrospinal fluid, with regulatory clearance (e.g., FDA 510(k), CE marking), and generally limited to those intended specimen types. Although body fluids are often considered ultrafiltrates of plasma, their chemical characteristics may differ and can significantly affect assay performance. For example, pH differences can alter enzymatic reactions and may even lead to enzyme inactivation, while variations in protein or lipid concentrations may also introduce exclusion effects in indirect ion-selective electrode (ISE) measurements.
To ensure that results from testing in nonstandard body fluids are reliable, laboratories accredited by the College of American Pathologists (CAP) are required to evaluate and demonstrate that matrix effects do not compromise result accuracy when these assays are applied to nonstandard body fluids. Standard validation procedures typically include spike-and-recovery experiments and calculation of bias introduced by alternate matrices (1,2). In practice, this regulatory and operational requirement can become burdensome, particularly when each analyte-fluid combination is approached sequentially, and laboratories may spend substantial time and resources planning and completing these studies. Our recent work was motivated by this very problem: how can laboratories assess body fluid matrix effects in a practical, analytically sound, and scalable manner?
We developed a multiplexed approach for rapidly assessing body fluid matrix effects (3). Specifically, we spiked body fluids using pooled patient plasma containing high concentrations of multiple analytes. By doing so, multiple analytes can be assessed simultaneously with far fewer manual steps than in a traditional “one analyte per spike” approach. We applied this design to 15 chemistry analytes across 5 body fluid types and used confirmatory dilution studies to further assess linearity and matrix effects. The findings were encouraging: matrix effects for all evaluated analytes fell within the predefined acceptance criterion of ±20%, and dilution linearity was strong overall (R² ≥ 0.90). However, not all analyte–fluid combinations behaved equivalently, with some exhibiting time-dependent instability, emphasizing that specimen stability and timing of analysis must be considered alongside recovery and linearity.
For clinical laboratories, the broader lesson is that evaluation of body fluid matrix effects does not have to be prohibitively labor intensive. In our experience, a multiplexed approach can provide a practical framework for meeting compliance expectations while conserving staff time and instrument resources. As laboratories continue to support increasingly diverse specimen types, many of which will not be validated by the manufacturer, efficient and well-designed approaches for evaluating matrix effects in these specimens will be essential for maintaining both regulatory compliance and the quality of patient results in the clinical laboratory.
References
- Block DR, Ouverson LJ, Wittwer CA, Saenger AK, Baumann NA. An approach to analytical validation and testing of body fluid assays for the automated clinical laboratory. Clin Biochem 2018;58:44-52.
- Owen WE, Thatcher ML, Crabtree KJ, Greer RW, Strathmann FG, Straseski JA, Genzen JR. Body fluid matrix evaluation on a Roche cobas 8000 system. Clin Biochem 2015;48:911-4.
- Ge M, Seely S, Kelner MJ, Fitzgerald RL, Suhandynata RT. A Rapid Approach for Assessing Body Fluid Matrix Effects. J Appl Lab Med. 2025;10:1552-63.
