| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Technical Briefs |
1 Department of Pathology and Immunology, Division of Laboratory Medicine, Washington University School of Medicine, 660 South Euclid Ave., Box 8118, St. Louis, MO 63110
aauthor for correspondence: fax 314-362-1461, e-mail gronowski{at}pathology.wustl.edu
Methods to quantify human chorionic gonadotropin (hCG) in serum are well established (1), but automated quantitative urine assays are not readily available. When the validity of point-of-care qualitative urine hCG results are called into question, a rapid quantitative method for urine hCG could be useful. In the present study, we have validated the Abbott AxSYM Total ß-hCG (Abbott Laboratories) assay (approved for serum use only) for use in the quantitative determination of urinary concentrations of total ß-hCG.
Recovery studies were performed by adding hCG (US Pharmacopeia) to urine from premenopausal, nonpregnant females. Recovery studies were carried out in quadruplicate in two separate experiments. Recovery was 99–112% at concentrations of 26–725 IU/L (Table 1
). Urine protein at concentrations up to 7.4 g/L had no effect on recovery (not shown).
|
Intra- and interassay imprecision was evaluated by adding hCG at three different concentrations into urine from a premenopausal, nonpregnant female. Intraassay imprecision (CV) was 4.0% at 31 IU/L, 3.9% at 181 IU/L, and 2% at 652 IU/L (n = 10 at each concentration). Interassay (total) imprecision (CV) for twice-daily assays over 7–10 days (of a separate set of urine samples with added hCG) was 6.8% at 30 IU/L, 4.5% at 175 IU/L, and 10% at 454 IU/L. This precision is similar to that seen in our laboratory for serum (not shown).
The minimum detection limit was evaluated by measuring total ß-hCG in urine from 10 premenopausal, nonpregnant females and 36 males. The mean hCG was 4.35 IU/L (range, 0–9.1 IU/L; SD, 1.8 IU/L). The minimum detection limit, calculated as the mean + 3 SD, was thus 9.7 IU/L.
We measured total ß-hCG in duplicate in two experiments in samples with 0–40 IU of hCG added per liter. Results were linear from 10 to 40 IU/L, but deviated >10% from the expected at concentrations at concentrations <10 IU/L (Fig. 1A
). Therefore, we considered the lower limit of the reportable range to be 10 IU/L. For serum, our laboratory determined this limit to be 2 IU/L.
|
Linearity was evaluated by adding hCG to urine from a premenopausal, nonpregnant female at concentrations from 0 to 1070 IU/L. Total ß-hCG was measured in duplicate. The assay was linear to 535 IU/L, deviating >10% from the expected at concentrations >535 IU/L (Fig. 1B
). For serum, our laboratory found the assay to be linear to 800 IU/L.
There are several caveats regarding the measurement of hCG in urine. The first caveat is that urine contains regular, intact hCG; nicked hCG; hyperglycosylated hCG; free ß-hCG; and a significant amount of ß-core fragment (1). Many assays designed to measure serum hCG, including the Abbott AxSYM, are unable to detect the ß-core fragment (1). Therefore, it is possible that some urine samples may have intact hCG concentrations too low to be detected by the Abbott AxSYM method, when at the same time, the ß-core fragment concentrations are increased. However, this inability to detect the ß-core fragment is also a problem with the point-of-care hCG devices: they detect only intact hCG. The second caveat is that urinary hCG concentrations can vary significantly depending on fluid intake. Therefore, a serum hCG-positive individual may test negative on a urine hCG test because of very dilute urine. This is one of the limitations of testing urine to confirm pregnancy, especially on random samples. A positive is useful, but a negative must be interpreted with caution. For the purposes of confirming point-of-care hCG devices, the Abbott AxSYM should be sufficient because both methods are subject to the same set of limitations.
The Abbott AxSYM Total ß-hCG assay can be used to quantify urine ß-hCG as low as 10 IU/L. The point-of-care urine hCG devices used in laboratories detect hCG concentrations 
20 IU/L (2). Therefore, the quantitative method described appears to provide an adequate detection limit for confirming results from point-of-care devices. Furthermore, it showed excellent precision and is fully automated, making it an easy method to use in the laboratory.
Parts of this work were presented at the AACC Annual Meeting, July 28–August 1, 2002.
References
The following articles in journals at HighWire Press have cited this article:
|
|
 |
|
  A. M. Halldorsdottir, M. O. Carayannopoulos, M. Scrivner, and A. M. Gronowski Method Evaluation for Total {beta}-Human Chorionic Gonadotropin Using Urine and the ADVIA Centaur Clin. Chem., August 1, 2003; 49(8): 1421 - 1422. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
