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Answering your questions.

Cost-effective test for E coli

Q Our six-physician pediatric office sends stool cultures to the local hospital, which plates them on particular media. If we are looking for Escherichia coli, it is plated on an extra medium--MacConkey agar with sorbitol. If E coli growth is suspected, then a latex test is done for ID. Two outside labs servicing our area do not culture for E coli. They run an ELISA test for Shiga toxins; they do not grow it on MacConkey agar with sorbitol. According to one of these labs, this is a more efficient method to identify E coli. We are interested in the fastest, most cost-effective, and most accurate method for identifying patients who have E coli.

A The following answer is derived from a recent publication from the Centers for Disease Control and Prevention (CDC). (1) Escherichia coli 0157:H7 and other strains that produce Shiga toxin are collectively known as Shiga toxin-producing E coli (STEC). The CDC publication highlights the importance of culture confirmation of non-culture methods of Shiga toxin (Stx) detection (as falsely positive Stx results can delay the identification of an outbreak due to another organism).

E coli 0157:H7 does not ferment sorbitol and can be isolated on media developed to detect the sorbitol-negative organisms; however, most other strains of STEC ferment sorbitol and are phenotypically indistinguishable from other non-toxin-producing E coli isolates. Stx detection methods have been developed (e.g., enzyme immunoassay [EIA] and polymerase chain reaction [PCR]) that will allow for the screening of stools for STEC, regardless of serotype.

The CDC recommends that all labs consider including STEC 0157 in their routine enteric panel and that, if culture is not automatically performed along with Stx testing, all positive Stx specimens have culture confirmation. Six specific recommendations in this CDC publication are:

1. Healthcare providers should notify clinical diagnostic labs when STEC 0157 infection is suspected (e.g., because of bloody diarrhea or hemolytic uremic syndrome) so that appropriate testing methods can be applied.

2. Clinical diagnostic labs should strongly consider including STEC 0157 in their routine bacterial enteric panel (with Salmonella, Shigella, and Campylobacter).

3. To best identify all STEC infections, screen all stool samples submitted for routine enteric bacterial testing for Shiga toxins using EIA or PCR. Ideally, the clinical diagnostic lab should culture simultaneously for STEC 0157 (e.g., on sorbitol MacConkey agar). Simultaneous culture facilitates rapid diagnosis and treatment of patients with STEC 0157 infection and rapid subtyping by public-health labs; such rapid action is most important when the index of clinical suspicion for STEC 0157 is high.

4. Clinical diagnostic labs that use a Stx EIA but do not perform simultaneous culture for STEC 0157 should culture all Stx-positive broths for STEC 0157 as soon as possible and rapidly forward these isolates to a state or local public-health lab for confirmation and subtyping.

5. When a Stx-positive broth does not yield STEC 0157, the broth culture should be quickly forwarded to a local state public-health laboratory for identification of non-0157 STEC.

6. State and local public-health labs should confirm the presence of Stx in broths sent from clinical labs and attempt to obtain an STEC isolate. All non-0157 STEC isolates should be sent by public-health labs to CDC for confirmation and further characterization.

In answer to your question, keeping in mind the recommendations of CDC, "the best way" to identify all STEC infections is to screen all stool samples submitted for routine enteric bacterial testing for Stx using EIA or PCR; and culture the specimens simultaneously for STEC 0157 (e.g., on sorbitol MacConkey agar). It seems that neither of the labs you mentioned is following this "ideal" recommendation--understandable, as this is a relatively new recommendation, and one which could significantly increase costs for the patient and the lab performing the testing.

If you indicate to the lab staff that you suspect STEC 0157, and they culture on appropriate media looking for the organism and appropriately identify it, this is within the CDC recommendations listed above. If you send a specimen suspected of containing STEC 0157 to a lab that performs the EIA or PCR to test for Stx, then you should ensure that they also have culture confirmation for positive Stx tests or that they send these specimens to a public-health lab for culture confirmation; this would then also be within current CDC recommendations.

Additional recommendations from the CDC concerning testing of stool specimens for Stx-producing E coli are anticipated this year.

--Susan E. Sharp, PhD (DABMM)

Director of Microbiology

Kaiser Permanente

Pathology Regional Laboratory;

Associate Professor

Oregon Health and Science University

Portland, OR


(1.) Centers for Disease Control and Prevention. Importance of culturing confirmation of Shiga toxin-producing E coli infection as illustrated by outbreaks of gastroenteritis: New York and North Carolina, 2005. MMWR Morb Mortal Wkly Rep. September 29, 2006;55(38):1042-1045.

Factor Xa vs. APTT

Q I am a nurse with patients receiving heparin therapy. Why would you use a Factor Xa test vs. an APPT test? Please answer at a very basic, nursing level.

A If the patient is on low molecular-weight heparin (LMWH), the APTT is not a good measure of therapy as it will not elevate in relationship to the patient's LMWH level. In this instance, an anti-FXa LMW heparin assay should be performed if monitoring is needed.

If the patient is on unfractionated heparin (UFH), the laboratory should have an APTT range established to monitor heparin. The APTT is considered a surrogate marker for heparin levels, but there are many conditions that will elevate or even shorten the APTT.

If the baseline APTT is elevated or shortened, this can interfere with the ability of the APTT to correlate with the UFH level in the patient. The correlation, in fact, may not be very good. The anti-FXa heparin level should correlate better with the patient's heparin level.

--Dorothy M. Adcock, MD

Medical/Laboratory Director

Esoterix Coagulation

Englewood, CO

Microscope headache

Q In 33 years of lab work and training, this is the first time I have encountered an employee who gets a bad headache every time she uses the microscope. After only two urine microscopic exams, she has to quit. She has astigmatism. She can focus and identify objects correctly and has tried working with and without her glasses. Any ideas?

A Most people suffer in silence, but many suffer with similar challenges. There are probably very few optometric offices that go even two days without seeing at least one person complaining of issues like the ones you describe. A long list of conditions make such a job hostile and unrewarding and, without proper attention and correction, may force a person into another job or career.

Usually, the greater the farsightedness (hyperopia) and the older the person, the greater the difficulty. That is not to say that some individuals with moderate hyperopia between ages 30 to 40 would not have a similar struggle.

The questioner comments that many have astigmatism and do just fine. Again, there is a limit as to how long a person with moderate to large amounts of astigmia can concentrate on a detailed task without some avoidance and/or fatigue that can lead to an eye-tension-related headache. She wears lenses but are they a) up to date and b) adjusted correctly for the tasks of microscopy? If she is using a binocular microscope and one eye is over- or under-corrected, that alone could cause the difficulties.

Aging-eye syndrome (presbyopia) can be an issue as early as age 38, particularly if visual demands are unusually taxing. Even if this employee wears the best astigmatic correction possible and presbyopia is setting in, the person may possibly need a near-point correction, in addition to the astigmatic prescription. All kinds of people wear task-specific lens prescriptions that differ from what they would use in driving a car.

Some people (even in their 20s) with and without astigmatism simply cannot sustain prolonged accommodation at a near-point task (accommodative insufficiency) without unusual fatigue, even if they enjoy the task. Specialized visual therapy helps, while others benefit with a task-specific prescription as with the presbyopes.

Some people have moderate to slight near-point muscle imbalances that can trigger headache-type situations as you describe. Some have what is called an overconvergence pattern--often associated with the hyperopia condition--where even 15 to 20 minutes in intense reading environments causes a headache. Another type of muscle imbalance called underconvergence also causes fatigue and, if it is too significant, can cause a person to either give up or take a pain medication. A third type of muscle imbalance is called a hyperphoria or vertical-muscle imbalance, which is more subtle but can cause the same effects.

If it has been more than a year since the last exam and/or prescription update, she needs to return and explain the occurring issues to a trusted practitioner. If the practitioner has done the best he knows how to do and her problem still continues, she needs a second opinion; something uncorrected and/or undetected is occurring. This should be alleviated and addressed by someone who best understands functional near-point strain issues. Not all practitioners address this issue the same way.

--Terrance Hohner, OD

Barbur Boulevard Optometric Clinic

Portland, OR


Daniel M. Baer, MD, is professor emeritus of laboratory medicine at Oregon Health and Science University in Portland, OR, and a member of MLO's editorial advisory board.
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Title Annotation:Tips from the clinical experts; identifying shiga toxins in escherichia coli
Author:Baer, Daniel M.
Publication:Medical Laboratory Observer
Geographic Code:1USA
Date:Jan 1, 2009
Previous Article:Addressing management issues.
Next Article:Diversity invites sensitivity.

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