Desiccated Coconut as a Quality-Control Material in Fecal Fat Measurements

Clinical Chemistry Laboratory, Altnagelvin Hospital, Glenshane Road, Londonderry,, N Ireland BT47 1SB
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To the Editor:

The measurement of fecal fat excretion (1) remains an important test in the diagnosis of fat malabsorption despite the development of alternative biochemical investigations, such as the butterfat test (2), steatocrit (3), and triolein breath tests (4), and alternative diagnostic strategies in gastroenterology. However, the measurement of fecal fat excretion has suffered from poor reproducibility because of both preanalytical and analytical factors. Although the former may be minimized by the use of standardized study protocols, there is no universally employed method of internal quality control of the analytical phase (5). Although modern analytical techniques such as near infrared analysis (6) offer improved analytical performance, most laboratories performing fecal fat measurements still use chemical methods described in the 1950s. These methods involve the hydrolysis of fecal fat, followed by extraction of the liberated fatty acids and measurement by titrimetric or gravimetric methods (1).

The ideal quality-control material for fecal fat assays would assess performance of the assay as a whole (hydrolytic, extraction, and titration steps), would be stable on storage, would be free of major health and safety problems, and would be easily and conveniently handled by laboratory staff. Anecdotal reports suggest that approaches to internal quality control are varied and range from control of the titration step alone to the use of single fats such as triolein, pure fat solutions such as oils, or fat-rich materials such as peanut butter. Although repeat analysis of a feces sample with the next patient sample may also be performed, health and safety considerations limit the use of stored feces as a method of monitoring long-term assay performance.

We have explored the use of flaked desiccated coconut as a suitable quality-control material. Desiccated coconut is rich in fat (~53 g/100 g) comprising mainly saturated fat (C12:0, C14:0, C8:0, and C16:0) together with C18:1 and C18:2 unsaturated fatty acids. Flaked desiccated coconut for culinary use was obtained from a commercial source. The coconut was mixed well, divided into 0.2-g aliquots, and frozen at -30 °C until analysis. This sample was added to a boiling tube, and the fat content was measured by the method of van de Kamer (1), which is our standard fecal fat method. Over a 4-month period this material gave, in the final alkali titration step of the assay, a titration volume of 1.92 mL (0.16) [mean (SD); n = 12], i.e., a coefficient of variation of 8.4%. There was no evidence of deterioration in the material over this period.

Flaked desiccated coconut is easy to handle, poses no health and safety problems, and is suitable for long-term storage. It allows quality control of all three analytical phases of the assay, and because it involves the extraction of a range of fatty acids from a biological matrix, albeit a somewhat different matrix than feces, it is nevertheless a more biologically suitable control material than single fat or pure fat materials. We therefore propose that desiccated coconut is a simple, cheap, and convenient material for monitoring the analytical performance of fecal fat assays. Furthermore, this material might prove suitable for use in external proficiency testing schemes.

References

1. van de Kamer JH. Total fatty acids in stool. Seligson D eds. Standard methods of clinical chemistry 1958;Vol. 2:34-39 Academic Press New York. .
2. Harper GD, Wheeler DC, Wicks AC. Butterfat absorption—a valuable screening test in malabsorption. Post Med J 1994;70:23-26.
3. Sugai E, Srur G, Vazquez H, Benito F, Maurino E, Boerr LA, Bai JC. Steatocrit: a reliable semiquantitative method for the detection of steatorrhoea. J Clin Gastroenterol 1994;19:206-209. [Medline] [Order article via Infotrieve]
4. Bhattia SK, Bell TK, Love AHG, Montgomery DAD. An evaluation of a test using ¹⁴C labelled triglyceride in the diagnosis of steatorrhoea. Ir J Med Sci 1969;2:545-552.
5. Theodossi A, Gazzard BG. Have chemical tests a role in the diagnosis of malabsorption?. Ann Clin Biochem 1984;21:153-165.
6. Beckers O, Postma C, Lombarts AJ. Determination of faecal fat by near infrared absorption spectroscopy. Eur J Clin Chem Clin Biochem 1995;33:83-86. [ISI][Medline] [Order article via Infotrieve]