Answering your questions on rapid drugs-of-abuse tests, Apt testing, the etiology of diarrhea, and restaining of fluorochrome slides.

Rapid drugs-of-abuse tests

Q How do the rapid tests for drugs of abuse, based on cup or cassette format, compare with the tests run in a clinical laboratory? Under what circumstances are the rapid tests appropriate?

A There are a number of point-of-care, or rapid tests, for drugs-of-abuse testing available currently; the most commonly used is the Triage (BioSite Diagnostics, San Diego, CA), but other rapid testing formats [including EZ-Screen (Diagnostix Ltd., Toronto, Canada) and Ontrak (Roche Diagnostics, Indianapolis, IN)] are also available. All of these use immunoassays, similar to the more commonly used automated instrument assays. I have referenced several comparison studies of these assays with automated immunoassays and GCMS confirmatory testing methods. [1-4] In general, the analytical performance of these assays was similar to that of more traditional techniques. One study found that EZ-Screen did not appear to adhere to standard or specified cutoff concentrations, reporting as positive samples that were below the specified cutoff values. [4] Another study by Kranzler et al. found that Triage gave less accurate testing results when used by nonlaboratory personnel, [2] but de la Torre et al. found test performa nce was not affected by the skill of the operators. [3]

Based on this information, it would appear that such rapid testing could be used in situations where results are needed urgently and cannot be provided by other techniques. This may occur in a smaller laboratory that has limited staffing and no instrumentation that can perform standard immunoassays, or for point-of-care testing in an emergency unit where rapid turnaround time is needed. If testing is to be performed by nonlaboratory personnel, comparison with standard laboratory techniques should be performed before instituting testing, particularly with specimens having results near the cutoff concentrations. The personnel doing testing should be made aware of the potential for false-positive results with any immunoassay method as well as the need for confirmatory testing when lab results conflict with patient history or physical examination, or in all cases, dependent on the confirmatory testing policies of the laboratory.

D. Robert Dufour, MD

Chief of Pathology and Laboratory Medicine

Veterans Affairs Medical Center

Washington, DC

References

(1.) Ferrara SD, Tedeschi L, Frison G, et al. Drugs-of-abuse testing in urine: Statistical approach and experimental comparison of immunochemical and chromatographic techniques. J Anal Toxicol. 1994;18:278-291.

(2.) Kranzler HR, Stone J, McLaughlin L. Evaluation of a point-of-care testing product for drugs of abuse; testing site is a key variable. Drug Alcohol Depend. 1995;40:55-62.

(3.) de la Torre R, Domingo-Salvany A, Badia R, et al. Clinical evaluation of the Triage analytic device for drugs-of-abuse testing. Clin Chem. 1996;42:1433-1498.

(4.) Crouch DJ, Frank JF, Farrell LJ, Karsch HM Klaunig JE. A multiple-site laboratory evaluation of three on-site urinalysis drug-testing devices. J Anal Toxicol. 1998;22:493-502.

Apt test

Q We occasionally get an order for an Apt test to determine if the blood in a neonate's emesis or stool is maternal or fetal in origin. My understanding is that this is not a very reliable test. A recent reference (McRury JM, Barry RC. A modified Apt test: A new look at an old test. Ped Emerg Care. 1994;10:189) indicates that the test has a low sensitivity, and results should be interpreted with caution. Is this test still acceptable practice? If not, is there a replacement test?

A I was not acquainted with this test had to do some searching to get an answer. The test was developed by Dr. Apt in the 1950s, [1] hence the name. The test is based on the denaturation of hemoglobin by alkaline solutions, the same technique that is used to detect fetal cells in the maternal circulation. Adult hemoglobin is less resistant to 10% NaOH than is fetal hemoglobin. When NaOH is added to the test specimen, adult hemoglobin turns a brown color whereas fetal hemoglobin remains pink.

According to information from a couple of university-based neonatal intensive care units, the test is rarely, if ever, ordered and then only by the obstetrical staff. The NaOH reagent is made available to the resident or staff physicians who wish to do the test. The initial color of the test specimen can vary, and interpretation can be difficult because it is so subjective. Appropriate controls would help in the interpretation.

Incidentally, the Apt test is not included in the CPT code listing of reimbursable laboratory tests.

In reviewing the original report, there were 7 cases in which the test was negative and the peripheral blood hemoglobin levels were also stable. In 5 of the 6 cases with a positive test (fetal hemoglobin present in the stool) there was also a significant drop in the neonate's hemoglobin level. [1] So perhaps point-of-care monitoring of hemoglobin levels might be a reasonable substitute for the Apt test.

John A. Koepke, MD

Professor Emeritus of Pathology

Duke University Medical Center

Durham, NC

Reference

(1.) Apt L, Downey WS. "Melena" neonatorum: The swallowed blood syndrome. A simple test for the differentiation of adult and fetal hemoglobin in bloody stools. F Ped. 1955;47:6-12.

Etiology of diarrhea

Q Can bacterial species other than the commonly recognized stool pathogens cause diarrhea? What methods should be used to confirm their role in the diarrheal episode?

A On rare occasion, bacteria such as enterococci and gram-negative rods (Klebsiella, Proteus, Enterobacter, Citrobacter, and Pseudomonas spp) have been implicated in cases of bacterial diarrhea." [1,2] Gram-negative rods responsible for diarrheal outbreaks have been shown to produce enterotoxins, at least transiently. If identification of the causative agent is required for the management of the patient, the laboratory should first rule out all of the more common stool pathogens. After the common causes of diarrhea have been eliminated, the predominant enteric species can be tested for the production of enterotoxin. Most clinical microbiology laboratories do not have this expertise and, therefore, the questionable isolate is usually sent to a reference laboratory that specializes in this type of testing.

David L Sewell, PhD. ABMM 

Director of Microbiology

Veterans Affairs Medical Center

Portland, OR

References

(1.) Tauxe RV, Hughes JM. Food-home disease. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 4th ed. New York: Churchill Livingstone; 1995: p. 1012-1024.

(2.) Isenberg HD, D'Amato RF. Indigenous and pathogenic microorganisms of humans. In: Murray PR, Baron EJ, Pfaller MA, Tenover FC, Yolken RH, eds. Manual of Clinical Microbiology. 6th ed. Washington, DC: American Society of Microbiology; 1995: p. 5-18.

Restaining of fluorochrome slides

Q Can fluorochrome stains be restained by one of the carbolfuchsin methods?

A The fluorochrome stains are the preferred method to detect mycobacteria in clinical specimens because you use a lower magnification to screen the slide. This allows the screener to view more of the slide in a shorter period of time. When suspected mycobacteria are identified in the fluorochrome stained smear, the slide can be restained with the Zeihl-Neelsen or Kinyoun acid-fast method to confirm acid-fast bacillus or to store the slide for future reference. [1]

David L. Sewell, PhD

Reference

(1.) Kent, PT and Kubica, GP. Public Health Mycobacteriology: A Guide for the Level III Laboratory. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control; 1985.

Daniel M. Beer is professor emeritus of laboratory medicine at Oregon Health Sciences University in Portland, OR, end a member of MLO's editorial advisory board.