Q. Preadmission (or on admission) testing
for methicillin-resistant Staphylococcus aureus, or MRSA, with
molecular (polymerase chain reaction) or conventional culture has decreased
hospital infection rates when the detection of MRSA is acted on within
a short time after admission (with appropriate isolation). Are patients
colonized with MRSA (for example, nasal carriers) simply discharged to
return to their nursing homes or other places of residence without treatment,
or is some form of therapy, local or otherwise, indicated?
A. The primary purpose of screening for MRSA
on admission is to quickly identify colonized patients so they can be
placed into contact isolation to stop the spread of MRSA to additional
patients. The rationale for decolonizing those who already harbor MRSA
in their nose is based on the hypothesis that doing so will prevent future
disease in those already colonized by removing MRSA. However, decolonization
is of unproven benefit at this time. Thus, which strategy to choose is
up to the individual health care organization. Some may elect to perform
only admission surveillance. Others may choose, as does Evanston (Ill.)
Northwestern Healthcare, to decolonize patients who are found to be nasally
MRSA positive by using a simple regimen of mupirocin ointment rubbed into
the anterior nares twice a day for five days, accompanied by a chlorhexidine
bath or shower on days one, three, and five of the mupirocin application.
For ENH, the strategy of active surveillance followed by contact isolation
and decolonization of MRSA carriers has led to an 80 percent reduction
of MRSA bloodstream infections in the first year of our program.
Lance R. Peterson, MD
Director of Microbiology and Infectious Diseases Research
Department of Pathology and Laboratory Medicine
Evanston Northwestern Healthcare
Q. A few of our doctors have expressed concern
about some of their patients' successive hematocrits, which are drawn
several times during a 24-hour period. I know there are a number of preanalytic
factors that can influence hematocrit, ranging from the patient's physical
state (renal, glucose, and hydration status) to basic phlebotomy technique.
For example, one patient had a 25 percent hematocrit result at 8 AM, a
27 percent hematocrit result at 4 PM, and then was back down to a 25 percent
hematocrit result at midnight—with no transfusion in between.
Can you provide information that I can
use to try to convince two clinicians that they must consider preanalytic
factors? We repeated all samples again on our Advia 2120 hematology systems.
We also do frequent manual-spun hematocrit checks on the Advia result
when we have a CHCM versus MCHC flag. When we run quality control material,
it is always within one standard deviation of the target value for the
QC product as specified by its manufacturer. We also QC all parameters
three times per day, so the analytic part is fine. Our allowable Advia
reproducibility is a hematocrit result that is within a range of two percent
above or two percent below the first determination. All samples retested
were within a one percent hematocrit result of the original test results.
A. Several preanalytic factors can affect hematocrit
determination, which could account for the observed slight variation in
hematocrit results. One is the state of the intravascular volume of the
patient. Hydration changes and fluid shifts can affect intravascular volume
and lead to changes in hematocrit as red blood cells are confined to the
intravascular space, whereas the water and electrolytes in plasma are
not. Changes in the patient's state could also account for the small variation
reported. The source of the specimen can influence the result as well.
Because fluid shifts normally occur at the capillary level, capillary
hematocrits (obtained by finger prick) often yield a hematocrit two or
three units higher than a venous hematocrit (obtained by venipuncture).
Analytic variation could also account for the observed results. An instrument
operating correctly will repeat hematocrit values within three to five
percent. At the hematocrit described in the example, this would be about
one unit, making the observed variation within the reproducibility of
the instrument—for example, the 25 could have been a 26 or the 27
could have been a 26.
One caution should be noted in using spun hematocrits
when comparing to instrument-generated hematocrits: An instrument hematocrit
is a calculated value (the instrument measures the red blood cell count
and mean corpuscular volume), so it is actually the hematocrit with no
trapped plasma. Spun hematocrits have some degree of plasma trapping.
If the patient has red blood cell alterations that interfere with red
blood cell stacking, spun hematocrits can be multiple units higher than
the calculated hematocrit obtained by the instrument.
Robert Novak, MD
Department of Pathology
Medical Center of Akron
Chair, CAP Hematology and Clinical
Microscopy Resource Committee