Q&A column

There are several ways to evaluate the efficacy of peritoneal dialysis during therapy. One recommended metric is the Kt/V value for urea, in which K is the peritoneal urea clearance, t is time, and V is the volume of distribution of urea, which is approximately equal to the patient’s total body water volume.⁶ Formulas for calculating Kt/V range from simple ones requiring only pre- and post-blood urea nitrogen (BUN) to much more complex, full kinetic models.

The peritoneal equilibration test (PET), introduced in 1987, is another important tool for evaluating peritoneal membrane transport function because it takes into account the dialysate-to-plasma ratio of creatinine and glucose, ultrafiltration volume, and samples taken at multiple time points.^7,8 Various groups have advocated for modifying the traditional PET.^9,10

Returning to the reader’s question, one should consider, for example, that a glucose measurement using fluid drawn from the peritoneal cavity would have different implications in evaluating bacterial peritonitis than it would in a routine peritoneal equilibration test. Even for patients with underlying liver cirrhosis and ascites who are undergoing peritoneal dialysis, the timing of sample collection influences whether the fluid is composed primarily of ascites or dialysate. Similarly, catheterized urine has relevant differences compared to bladder washing fluid, even though they are obtained from essentially the same anatomic location. Therefore, how the laboratory reports testing results can materially influence how the data are ultimately being interpreted. Without a notation or disclaimer, a result otherwise listed as peritoneal fluid in the medical record would likely be assumed to have been ascites fluid. In the scenario presented by the reader, documenting the fluid sample based on its primary composition or type (i.e. dialysate) instead of by its anatomic source provides key clinical context to the patient’s care team. However, if possible, documenting the fluid source and fluid type is optimal.

Finally, a laboratory should review potential regulatory compliance issues in relation to body fluid testing. Because few manufacturers offer commercial FDA-cleared body fluid tests, laboratories must often develop their own extensive validation methods to provide testing for the variety of body fluids they receive.¹¹ As laboratorians, we must be cognizant of the potential for matrix interference when we evaluate requests for body fluid testing or alternative matrices and ensure that we have completed the necessary due diligence to demonstrate analytical validity.

1. Himmelfarb J, Ikizler TA. Chronic Kidney Disease, Dialysis, and Transplantation. 4th ed. Elsevier; 2018.
2. Runyon BA. Care of patients with ascites. N Engl J Med. 1994;330(5):337–342.
3. Wright AA, Bohlke K, Armstrong DK, et al. Neoadjuvant chemotherapy for newly diagnosed, advanced ovarian cancer: Society of Gynecologic Oncology and American Society of Clinical Oncology Clinical Practice Guideline. Gynecol Oncol. 2016;143(1):3–15.
4. Runyon BA, Montano AA, Akriviadis EA, Antillon MR, Irving MA, McHutchison JG. The serum-ascites albumin gradient is superior to the exudate-transudate concept in the differential diagnosis of ascites. Ann Intern Med. 1992;117(3):215–220.
5. National Kidney Foundation. What is peritoneal dialysis? https://www.kidney.org/content/what-peritoneal-dialysis
6. Peritoneal Dialysis Adequacy Work Group. Clinical practice guidelines for peritoneal dialysis adequacy. Am J Kidney Dis. 2006;48(suppl 1):S98–S129.
7. Karl ZJT, Khanna ONR, Leonor BFP, Ryan P, Moore HL, Nielsen MP. Peritoneal equilibration test. Perit Dial Int. 1987;7(3):138–148.
8. Pannekeet MM, Imholz AL, Struijk DG, et al. The standard peritoneal permeability analysis: a tool for the assessment of peritoneal permeability characteristics in CAPD patients. Kidney Int. 1995;48(3):866–875.
9. Smit W, Langedijk MJ, Schouten N, van den Berg N, Struijk DG, Krediet RT. A comparison between 1.36% and 3.86% glucose dialysis solution for the assessment of peritoneal membrane function. Perit Dial Int. 2000;20(6):734–741.
10. La Milia V, Di Filippo S, Crepaldi M, et al. Mini-peritoneal equilibration test: a simple and fast method to assess free water and small solute transport across the peritoneal membrane. Kidney Int. 2005;68(2):840–846.
11. College of American Pathologists. COM.40620 Body fluid analysis. In: All common checklist. Sept. 22, 2021.

Brian Naidrin Chang, MD
Assistant Professor
Department of Laboratory Medicine
University of Texas MD Anderson Cancer Center
Houston, Tex.
Member, CAP Clinical Chemistry Committee

Q. What types of materials (for example, QC materials, patient samples, or both) can be used to check new reagent lots on my chemistry analyzer? We have three chemistry analyzers of the same model. Do we need to perform reagent lot studies on all three?

A.Your question relates to the CAP checklist COM.30450 New Reagent Lot and Shipment Confirmation of Acceptability—Nonwaived Tests. The checklist says, “New reagent lots and shipments are checked against previous reagent lots or with suitable reference material before or concurrently with being placed in service.”

The note in the checklist requirement indicates there are several materials that can be used, but patient samples are preferred. The CAP does not require that you use both patient samples and quality control material. It is up to your laboratory director to determine which material to use and the extent of the testing.

For quantitative testing, the checklist note provides the following list of suitable reference materials for checking new reagent lots and shipments:

• Patient specimens tested on a previous lot.
• Reference materials or QC products provided by the method manufacturer with method-specific and reagent lot-specific target values.
• Proficiency testing materials with peer group-established means.
• QC materials with peer group-established means based on interlaboratory comparison that is method specific and includes data from at least 10 laboratories.
• Third-party general-purpose reference materials if commutable with patient specimens for the method (per package insert or method manufacturer).
• QC material in use with the current reagent lot to check a new shipment of the same reagent lot. (There should be no change in potential matrix interactions with use of the same lot number of reagent and QC material.)

You need to perform this new reagent lot-to-lot verification using only one of your three analyzers. A separate checklist requirement (COM.04250) says, “If the laboratory uses more than one nonwaived instrument/method to test for a given analyte, the instruments and methods are checked against each other at least twice a year for comparability of results.”

Clinical and Laboratory Standards Institute. EP26-A: User Evaluation of Between-Reagent Lot Variation; Approved Guideline, 1st ed.; 2013.

Clinical and Laboratory Standards Institute. EP31-A-IR: Verification of Comparability of Patient Results Within One Healthcare System; Approved Guideline (Interim Revision); 2012.

College of American Pathologists. COM.04250 Comparability of instruments and methods—nonwaived testing. In: All common checklist. Sept. 22, 2021.

College of American Pathologists. COM.30450 New reagent lot and shipment confirmation of acceptability–nonwaived tests. In: All common checklist. Sept. 22, 2021.

Miller WG, Myers GL, Rej R. Why commutability matters. Clin Chem. 2006;52(4):553–554.

Wendy Parpart, MT(ASCP)
Senior Inspection Specialist
CAP Accreditation Programs
Northfield, Ill.