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CAP Home > CAP Reference Resources and Publications > CAP TODAY > CAP TODAY 2007 Archive > Mastering MRSA and more in latest CAP Surveys
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  Mastering MRSA and more in latest CAP Surveys

 

 

 

November 2007
Feature Story

Anne Paxton

The Oct. 17 estimate from the Centers for Disease Control and Prevention of the burden of invasive MRSA infections nationwide comes just as U.S. hospitals are starting to conduct preadmission rapid screening of patients for MRSA, or methicillin-resistant Staphylococcus aureus. Some state legislatures have even begun to require that screening.

Another laboratory test was pushed into the limelight on Aug. 16 when the Food and Drug Administration encouraged wider use of genetic testing to predict patients' response to warfarin, and approved the first commercial genetic test for warfarin sensitivity on Sept. 17.

With such strong signs that laboratories could be dealing with a long-term increase in both MRSA and warfarin genotyping test orders, some especially timely help is arriving from the CAP in the form of Surveys for those two screening tests.

Recently announced as additions to the 2008 Surveys catalog, these two Surveys will make it possible for hospital and reference laboratories to assess and improve their performance as they meet critical testing demands in infection control and anticoagulant therapy. "We're implementing these Surveys in anticipation of customer demand," says Kathleen Beavis, MD, chair of the College's Microbiology Resource Committee.

Though MRSA is not yet a reportable condition, the CDC has estimated the pathogen caused 94,000 U.S. infections and nearly 19,000 deaths in 2005. That compares to 17,011 people the CDC says died from the AIDS virus in the same year.

The MRSA figures, published in the Journal of the American Medical Association (2007;298:1763-1771), establish the first national baseline by which to assess future trends in invasive MRSA infection, the CDC says. Denise Cardo, MD, director of the CDC's Division of Healthcare Quality Promotion, said in an Oct. 16 statement that health care facilities need to make MRSA prevention a higher priority. "The closer we get to 100 percent compliance with CDC recommendations," she said, "the greater the impact on patient health and safety."

Though the CDC promotes "active surveillance" of patients to reduce MRSA infections, it has not recommended mandatory screening of all patients. However, the Veterans Health Administration has begun phasing in MRSA testing at all 155 of its medical centers, and it plans to test all patients starting next year.

The proper approach to MRSA infection control can be controversial, and many pathologists are wary of mandates to screen all patients. "The primary problem is transmission of nosocomial infections within the hospitals, and one of the organisms that has been implicated is MRSA. In the past it was predominantly an organism found in hospitals, but more and more often it's being found in the community," says Dr. Beavis, director of the microbiology and virology laboratory at Stroger Hospital of Cook County, Chicago.

Illinois, which passed a bill in August, is one of 14 states that require hospitals to identify people coming in who are carrying a MRSA infection and either segregate them or have infection control procedures made available to them. At Stroger Hospital, however, Dr. Beavis says, "we have plenty of challenges in infection control but nosocomial transmission of MRSA is not one of them. A hospital that knows it has a problem with nosocomial transmission of MRSA might want to adopt a screening program targeting its problem."

"Many hospitals have a totally different issue, such as transmission of VRE [vancomycin-resistant enterococci] or Clostridium difficile, and this special treatment for MRSA ties our hands. It doesn't give us the flexibility to put our resources where they would be of better use."

In an ideal world, in her view, state legislatures would not take away the judgment that pathologists, infection control specialists, and infectious disease physicians should be exercising. But the Surveys program is geared to respond to realities in the field.

"We did have an item in one of our Surveys testing the relatedness of strains; this item was geared toward laboratories that do epidemiologic investigations. This new Survey will address the needs of laboratories that screen for MRSA. We wanted to anticipate the demand for this type of Survey, and with the recent flurry in mandating MRSA screening, it made sense to break it out into its own Survey."

Warfarin genotyping testing challenges are included in the CAP's pharmacogenetics Survey (PGx), a new CAP product delivering proficiency evaluation for molecular diagnostic tests that predict patients' drug response.

About 2 million people start taking the oral anticoagulant warfarin each year in the U.S., yet warfarin is the second most common drug linked to emergency room visits. "It's one of the most common drugs associated with adverse drug reactions," says Charles S. Eby, MD, associate professor in the Department of Pathology and Immunology at Washington University, St. Louis. "Frankly, it's dangerous."

It takes advance planning and work to launch a new CAP Survey, and the College started to look into a warfarin genotyping Survey well before the FDA's recent relabeling decision, says Dr. Eby, who is one of the members of the College's Coagulation Resource Committee and Pharmacogenetics Working Group.

"When a new proficiency testing proposal is made, it is reviewed by the appropriate Survey committees, marketing department, and Council on Scientific Affairs," he says. Since new Surveys are usually launched only once a year, the planning process has to be completed in time for the catalog released in August.

"Another challenge is identifying a provider for the material we're sending out for the proficiency test," says Dr. Eby, who notes that the Surveys program often relies on the diagnostics industry to provide material. For the genetic Surveys, the CAP has tended to use the nonprofit Coriell Institute for Medical Research in Camden, NJ. "It provides a valuable function by storing cell lines for research as well as human DNA that is annotated for many clinically relevant polymorphisms," Dr. Eby explains. In addition, members of the Biochemical and Molecular Genetics Resource Committee confirm the expected genotypes in their clinical laboratories before Survey material is distributed.

Warfarin is not the first drug for which the FDA has changed the labeling to indicate a role for genetic testing before a person begins to take a medication, but the impact is likely to be greater since it is a commonly prescribed medication, Dr. Eby explains. For example, FDA labeling for Irinotecan, a chemotherapeutic for colon cancer, contains pharmacogenetic information and highlights the increased risk for severe neutropenia in patients who are homozygous for UDP-glucuronosyltransferase 1A1 (UGT1A1)*28 allele and are slower metabolizers of the active form of the drug.

The updated FDA labeling of warfarin provides similar pharmacogenetic information about warfarin and does not require that genetic testing be performed.

One of the problems with warfarin is that one dose does not fit all. "The dose has to be readjusted based on frequent monitoring of the International Normalized Ratio," Dr. Eby says.

"While clinical studies have helped define the therapeutic ranges for INR to minimize thrombotic and hemorrhagic complications, where there's been minimal progress until recently is in the ability to predict a therapeutic dose for individual patients when you're starting them on warfarin."

Determining the therapeutic dose at this point is a trial-and-error process, he says. "You typically start patients on 5 mg a day, then monitor the INR frequently and make adjustments as needed up or down. Therapeutic warfarin doses can vary from 0.5 mg up to 15 mg a day. That's a wide variability of dose in a drug that when you're over the therapeutic target, you increase the patient's risk of bleeding complications, and when you're under the target, you increase the risk of clotting complications."

Until now, the main approach to improving clinicians' ability to predict the patient's dose has been to identify clinical variables, including a person's age, weight or body surface area, and other medications they're taking that are known to inhibit or accentuate anticoagulant effects of warfarin. "These variables have been modestly helpful but probably only explain about 20 percent of the variation in dosing," Dr. Eby says.

"Recent improvements in predicting a patient's therapeutic warfarin dose are based on identification of polymorphisms in two genes whose products affect warfarin metabolism and warfarin inhibition of vitamin K cofactor activity," he says. Warfarin is predominantly metabolized by cytochrome P450 2C9 (CYP2C9) enzyme. Two CYP2C9 polymorphisms, *2 and *3, reduce the rate of warfarin clearance. The frequencies of *2 and *3 polymorphisms in Caucasians are approximately 11 percent and approximately seven percent, respectively, Dr. Eby says.

People who inherit these polymorphisms will be slow metabolizers of warfarin and will need a lower therapeutic dose, he adds.

Efficient synthesis of coagulation factors X, IX, VII, and II (prothrombin) requires vitamin K as a cofactor in a chemical reaction called gamma carboxylation. Vitamin K undergoes oxidation during this enzymatic step and must be converted back to its reduced form by the enzyme vitamin K reductase, or VKOR, in order to sustain coagulation factor production. "Warfarin competes with oxidized vitamin K for VKOR, resulting in lower levels of factors X, IX, VII, and II and a prolonged INR," Dr. Eby says. "Recent identification of the VKOR gene led to discovery of a common polymorphism in the promoter region of the gene that reduces gene expression and enzymatic activity, increases patients' sensitivity to warfarin, and lowers their therapeutic dose."

Knowing a patient's genetic status for three polymorphisms, CYP2C9 *2 and *3 and VKORC1 promoter -1639, allows physicians to account for about 30 percent of the variation of therapeutic dosage. Combining pharmacogenetic information with clinical details about a patient, such as age, body surface area, concurrent use of certain drugs such as amiodarone, and INR target, makes it possible to predict a patient's therapeutic dose with about 50 percent accuracy, Dr. Eby says.

Washington University and groups in Asia and Europe have confirmed this finding. "I think it's this information that led the FDA to make the decision to change the labeling," Dr. Eby says. Application of this pharmacogenetic information can be demonstrated, he says, by using a prospectively validated warfarin dosing algorithm, found at www.warfarindosing.org. A physician enters clinical, demographic, and CYP2C9 and VKORC1 genotype data, and the Web site estimates the therapeutic warfarin dose.

CYP2C9 and VKORC1 genetic testing will be of little use to the millions already taking warfarin. "Once you've established their dose, the genetic information is really not that important. We're really interested in the 2 million new people starting warfarin for the first time every year. The test makes prediction of the dose earlier and easier."

But one piece of the puzzle is still missing, he says, and the FDA has acknowledged this. "Using this genetic information to better estimate a patient's warfarin dose has not been shown to improve patient safety by reducing bleeding complications when starting warfarin, or even to improve how quickly a patient reaches and stays in therapeutic INR range compared to the trial-and-error method. Those studies have not been done yet." However, he adds, several institutions are in competition to lead a National Institutes of Health-sponsored prospective randomized study to address those questions.

"The FDA has said we think it's important enough to change the labeling to make clinicians aware they might want to test their patients for the single nucleotide polymorphisms in these two genes and start patients who have one or more of these SNPs on a lower warfarin dose, but we don't have the FDA saying you must do it."

Of the new CAP PGx Survey, Dr. Eby says, "I think any laboratory contemplating offering warfarin genotyping would want to participate in this CAP Survey-not only for quality control but also because it will be a great instrument to help laboratories learn how to report the information."

"On this Survey, obtaining the correct genotype is only part of the game. We ask participants to interpret the impact of the genotype on drug metabolism and activity and to make recommendations to clinicians regarding dosing modifications."

The Survey will be ungraded. "But," says Dr. Eby, "it's extremely helpful from an educational point of view for laboratories doing this testing and will help them communicate results to clinicians in a way most beneficial to the patient."

Further additions to the pharmacogenetics Survey are in the planning stages. Says Corrine Cagney, CAP product manager for microbiology/molecular diagnostics: "We're hoping to expand the PGx Survey to include other alleles as the pharmaceutical companies determine which alleles are predictive for their drugs. Abbott, GlaxoSmithKline, Pfizer, Roche, and Johnson and Johnson are all collaborating to decide what other genetic alleles will help determine drug response."

In addition to MRSA, mumps and Shiga toxin are two other important public health and safety tests that were mid-year additions to the College's Surveys catalog. The mumps Survey, designed for the new rapid test, came out last March and now has 260 participants. Tests for Shiga toxin, which is usually transmitted by undercooked contaminated meats and spread through incorrect hand-washing, are often sent to reference laboratories because few hospital labs perform the testing. The Shiga toxin Survey came out in August and has 132 participants.

In 2008, the CAP plans to add a QuantiFeron-TB Survey for tuberculosis. Like MRSA, Cagney notes, TB testing became high-profile after stories surfaced this year about public health hazards—in the case of TB, an infectious patient traveling by airplane.

All four Surveys are tied together as public health issues, she says. "We grouped them together not so much because they're differentiated, but because they are all things that affect the community." And they're good examples, Dr. Beavis says, of "how quickly the College can respond to the changing needs of the laboratory."


Anne Paxton is a writer in Seattle.
 
 
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