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Clinical Chemistry 51: 1907-1908, 2005; 10.1373/clinchem.2004.045468
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(Clinical Chemistry. 2005;51:1907-1908.)
© 2005 American Association for Clinical Chemistry, Inc.

Letters to the Editor

Pseudocholinesterase Activity in Organophosphate Poisoning after Storage of Unseparated Blood Samples at Room Temperature for 3 Weeks

Marc-Alexander von Mach1,a, Ludwig Sacha Weilemann1 and Philipp von Landenberg2

1 Division of Clinical Toxicology, and Poison Center II. Medical Department, and
2 Department of Clinical Chemistry, University of Mainz, Mainz, Germany

aAddress correspondence to this author at: Division of Clinical Toxicology and Poison Center, II. Medical Department, University of Mainz, Langenbeckstrasse 1, D-55131 Mainz, Germany. Fax 49-6131-17-6605; e-mail marcm{at}giftinfo.uni-mainz.de.


To the Editor:

Suppressed pseudocholinesterase activity is a well-established laboratory finding in patients with serious organophosphate poisoning (1). Recently, a 48-year-old man with suspected ingestion of methyl parathion died, and the postmortem examination was not indicative. After 3 weeks, an overlooked specimen was discovered that had been collected from the patient ~1 h after the suspected poisoning. The determination of pseudocholinesterase activity was requested. The blood sample, which showed complete hemolysis, was separated by centrifugation, and the pseudocholinesterase activity was determined. The result of 4.21 kU/L indicated the presence of only minor organophosphate poisoning without suppression of pseudocholinesterase activity.

Data regarding pseudocholinesterase activity in unseparated blood after storage at room temperature are rare, however, with the longest reported duration (48 h) showing only negligible differences (2). Because one would expect enzyme activities to be extensively changed after storage for 3 weeks at room temperature, we performed a limited study with samples from 10 randomly selected patients that had been submitted for the determination of pseudocholinesterase activity as part of our daily routine. Additionally, samples from 3 patients with serious organophosphate poisoning (parathion, patients 11 and 13; methyloxydemeton, patient 12) were included. Blood samples were collected in Monovette plastic tubes. One part of each sample was immediately centrifuged for 10 min at 2000g and 4 °C, and the pseudocholinesterase activity was measured by the colorimetric Roche Diagnostics® assay performed on a Hitachi 917 instrument. This test is based on the catalysis of butyrylthiocholine iodide to thiocholine and butyrate and is indicative of pseudocholinesterase activity (reference interval, 5.30–12.90 kU/L). The remaining portion of each sample was stored in a closed box at room temperature for 3 weeks. After this period, the samples were processed as described above and the pseudocholinesterase activity was measured. The study protocol was approved by the Institutional Review Board (Rheinland-Pfalz, Germany).

As shown in Table 1 , the pseudocholinesterase activity in the samples from randomly selected patients was decreased by a mean of 17.8 (4.3)% (maximum difference, +12.3%; median difference, –17.8%; minimum difference, –30.4%). Furthermore, we found no evidence for spontaneous in vitro recovery of pseudocholinesterase activity after storage at room temperature for 3 weeks; the activities in samples from the poisoned patients remained very low.


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  		Table 1. Pseudocholinesterase activity in samples from randomly selected patients (patients 1–10) and patients with organophosphate poisoning (patients 11–13) initially and after 3 weeks of storage at room temperature.

On the basis of this limited study, we conclude that the measurement of pseudocholinesterase activity in blood samples stored unseparated for several weeks at room temperature may still yield reasonable results for forensic questions. We are aware that, for forensic purposes in cases of organophosphate poisoning, the number of specimens collected from patients in our study was very limited. However, these poisonings are rare at our poison unit; a total of 9 patients were treated from January 2003 to December 2004. Furthermore, samples from 7 of these patients showed completely suppressed pseudocholinesterase activity (<0.1 kU/L), and only 2 patients had measurable values (0.39 and 1.30 kU/L) at presentation.

In view of the present findings, serious poisoning in the above case seems to be unlikely. However, another explanation for the missing suppression of pseudocholinesterase activity could be that parathion was not yet activated to paraoxon 1 h after the suspected poisoning.


References

  1. Wu AH, McKay C, Broussard LA, Hoffman RS, Kwong TC, Moyer TP, et al. National academy of clinical biochemistry laboratory medicine practice guidelines: recommendations for the use of laboratory tests to support poisoned patients who present to the emergency department. Clin Chem 2003;49:357-379.[Abstract/Free Full Text]
  2. Ono T, Kitaguchi K, Takehara M, Shiiba M, Hayami K. Serum-constituents analyses: effect of duration and temperature of storage of clotted blood. Clin Chem 1981;27:35-38.[Abstract/Free Full Text]




This Article
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Right arrow Drug Monitoring and Toxicology

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