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 coli0157: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
References
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.
such a job hostile and unrewarding and, without proper attention and correction, may force the person into another job or career.
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
