How to Test for MRSA: A Comprehensive Guide
Meticillin-resistant Staphylococcus aureus (MRSA) is a type of staph bacteria that is resistant to many antibiotics. Staph infections, including MRSA, can cause skin infections, pneumonia, bloodstream infections, and surgical site infections. Because MRSA is resistant to common antibiotics, it’s crucial to identify it quickly and accurately to ensure appropriate treatment and prevent its spread. This comprehensive guide will walk you through the different methods used to test for MRSA, providing detailed steps and instructions.
Understanding MRSA
Before delving into testing methods, it’s essential to understand what MRSA is and how it spreads.
* **Staphylococcus aureus:** A common bacterium that lives on the skin and in the noses of healthy individuals. Often, it doesn’t cause any problems.
* **Meticillin Resistance:** MRSA has developed resistance to beta-lactam antibiotics, such as meticillin (a type of penicillin), amoxicillin, and cephalosporins. This resistance makes it difficult to treat with standard antibiotics.
* **Transmission:** MRSA is typically spread through direct contact with an infected wound or contaminated surfaces. It can also be spread through respiratory droplets, although this is less common.
Who Should Be Tested for MRSA?
MRSA testing is not routinely performed on everyone. Testing is usually reserved for individuals who are at higher risk of carrying or developing a MRSA infection. This includes:
* **Hospitalized Patients:** Especially those in intensive care units (ICUs) or undergoing surgery.
* **Residents of Long-Term Care Facilities:** Nursing homes and assisted living facilities.
* **Individuals with Recurrent Skin Infections:** Boils, abscesses, or cellulitis.
* **Athletes:** Particularly those involved in contact sports.
* **Individuals with Indwelling Medical Devices:** Catheters, feeding tubes, or dialysis lines.
* **Household Contacts of MRSA-Infected Individuals:** To identify carriers and prevent further spread.
Methods for Testing MRSA
Several methods are used to test for MRSA, each with its own advantages and limitations. The most common methods include:
1. **Nasal Swab Culture:** This is the most widely used method for screening for MRSA carriage.
2. **Wound Culture:** Used to diagnose MRSA infections in wounds.
3. **Blood Culture:** Used to diagnose MRSA bloodstream infections.
4. **Molecular Tests (PCR):** Rapid and highly sensitive tests that detect MRSA DNA.
5. **Chromogenic Agar:** Selective media that allows for the visual identification of MRSA colonies.
1. Nasal Swab Culture: Step-by-Step Guide
The nasal swab culture is a simple and non-invasive procedure used to screen for MRSA in the nose, a common site of colonization.
**Materials Needed:**
* Sterile swab (usually a flocked swab)
* Sterile transport tube with transport media (e.g., Amies or Stuart’s media)
* Gloves
* Personal protective equipment (PPE) such as a mask (if indicated)
* Laboratory requisition form
* Biohazard bag for disposal
**Procedure:**
1. **Preparation:**
* Gather all necessary materials.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
* Put on gloves and any other required PPE.
* Label the sterile transport tube with the patient’s name, date of birth, date, and time of collection.
2. **Patient Positioning:**
* Explain the procedure to the patient to alleviate any anxiety.
* Ask the patient to tilt their head slightly back.
3. **Swab Collection:**
* Open the sterile swab package, being careful not to contaminate the swab.
* Insert the swab gently into one nostril, approximately 1-2 cm (0.5-1 inch).
* Rotate the swab against the nasal mucosa for 5-10 seconds, ensuring contact with the nasal lining.
* Remove the swab and repeat the process in the other nostril using the same swab.
* Avoid touching the outer skin of the nose with the swab.
4. **Swab Placement in Transport Media:**
* Immediately after collection, place the swab into the sterile transport tube.
* Ensure the swab is submerged in the transport media.
* Break off the excess portion of the swab stick, if necessary, so that the tube can be properly closed.
* Tightly seal the tube.
5. **Labeling and Documentation:**
* Double-check that the transport tube is correctly labeled with all required information.
* Complete the laboratory requisition form with the patient’s information, the source of the specimen (nasal swab), the date and time of collection, and any relevant clinical information (e.g., reason for testing, current medications).
6. **Storage and Transportation:**
* Store the specimen according to the laboratory’s instructions (usually at room temperature or refrigerated).
* Transport the specimen to the laboratory as soon as possible, ideally within 24 hours. Follow the laboratory’s specific transportation guidelines.
7. **Disposal:**
* Dispose of the gloves and any other contaminated materials in a biohazard bag.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
**Laboratory Analysis:**
* The laboratory will culture the swab specimen on selective media to allow for the growth of Staphylococcus aureus.
* If Staphylococcus aureus colonies grow, they will be tested for resistance to meticillin using antibiotic susceptibility testing methods, such as the disk diffusion method or the broth microdilution method.
* If the Staphylococcus aureus is resistant to meticillin, it will be identified as MRSA.
2. Wound Culture: Step-by-Step Guide
A wound culture is used to identify MRSA in infected wounds. Proper technique is essential to avoid contamination and ensure accurate results.
**Materials Needed:**
* Sterile swab (usually a flexible swab with a soft tip)
* Sterile transport tube with transport media (e.g., Amies or Stuart’s media)
* Sterile saline solution (for cleaning the wound, if necessary)
* Gloves
* Sterile gauze pads
* Personal protective equipment (PPE) such as a mask and eye protection (if splashing is likely)
* Laboratory requisition form
* Biohazard bag for disposal
**Procedure:**
1. **Preparation:**
* Gather all necessary materials.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
* Put on gloves and any other required PPE.
* Label the sterile transport tube with the patient’s name, date of birth, date, and time of collection, and the specific location of the wound.
2. **Wound Preparation:**
* Explain the procedure to the patient to alleviate any anxiety.
* If the wound is covered, carefully remove any dressings.
* Clean the wound with sterile saline solution to remove any debris or superficial contaminants. Avoid using antiseptic solutions, as they can interfere with bacterial growth.
* Dry the area around the wound with sterile gauze pads.
3. **Swab Collection:**
* Open the sterile swab package, being careful not to contaminate the swab.
* Gently insert the swab into the deepest part of the wound, avoiding contact with the skin around the wound.
* Rotate the swab over the wound surface, ensuring contact with any areas of pus, drainage, or granulation tissue. Collect the sample from the area that appears most infected.
* Apply enough pressure to collect an adequate sample, but avoid causing unnecessary pain or trauma to the patient.
4. **Swab Placement in Transport Media:**
* Immediately after collection, place the swab into the sterile transport tube.
* Ensure the swab is submerged in the transport media.
* Break off the excess portion of the swab stick, if necessary, so that the tube can be properly closed.
* Tightly seal the tube.
5. **Labeling and Documentation:**
* Double-check that the transport tube is correctly labeled with all required information.
* Complete the laboratory requisition form with the patient’s information, the source of the specimen (wound culture), the specific location of the wound, the date and time of collection, and any relevant clinical information (e.g., reason for testing, current antibiotic use).
6. **Storage and Transportation:**
* Store the specimen according to the laboratory’s instructions (usually at room temperature or refrigerated).
* Transport the specimen to the laboratory as soon as possible, ideally within 24 hours. Follow the laboratory’s specific transportation guidelines.
7. **Wound Care:**
* After collecting the specimen, dress the wound appropriately with sterile dressings.
* Provide the patient with instructions on how to care for the wound.
8. **Disposal:**
* Dispose of the gloves, dressings, and any other contaminated materials in a biohazard bag.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
**Laboratory Analysis:**
* The laboratory will culture the swab specimen on various media to identify all bacteria present in the wound.
* If Staphylococcus aureus is isolated, it will be tested for resistance to meticillin using antibiotic susceptibility testing methods.
* If the Staphylococcus aureus is resistant to meticillin, it will be identified as MRSA.
3. Blood Culture: Step-by-Step Guide
Blood cultures are used to diagnose MRSA bloodstream infections, which are serious and require prompt treatment.
**Materials Needed:**
* Sterile blood culture bottles (aerobic and anaerobic)
* Antiseptic solution (e.g., chlorhexidine gluconate or povidone-iodine)
* Sterile gloves
* Tourniquet
* Sterile gauze pads
* Sterile adhesive bandage
* Laboratory requisition form
* Blood collection set (needle, syringe or Vacutainer holder, and adapter)
* Personal protective equipment (PPE) such as a mask and eye protection (if splashing is likely)
* Biohazard bag for disposal
**Procedure:**
1. **Preparation:**
* Gather all necessary materials.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
* Put on gloves and any other required PPE.
* Inspect the blood culture bottles for any signs of damage or contamination. Do not use bottles that are cracked, cloudy, or have expired.
* Label the blood culture bottles with the patient’s name, date of birth, date, and time of collection.
2. **Patient Preparation:**
* Explain the procedure to the patient to alleviate any anxiety.
* Position the patient comfortably, usually in a supine or semi-recumbent position.
* Identify the venipuncture site. Avoid areas with visible lesions, scars, or hematomas.
3. **Skin Antisepsis:**
* Apply a tourniquet above the venipuncture site to make the veins more prominent.
* Cleanse the venipuncture site thoroughly with the antiseptic solution. Start at the center of the site and move outward in a circular motion. Allow the antiseptic solution to dry completely (usually 30-60 seconds) before proceeding with the venipuncture. Do not touch the site again after cleaning.
* Cleanse the tops of the blood culture bottles with alcohol pads before inoculation.
4. **Blood Collection:**
* Perform venipuncture using a sterile technique. Insert the needle into the vein at a 15-30 degree angle.
* Collect the appropriate amount of blood as specified by the blood culture bottle manufacturer (usually 8-10 mL per bottle for adults).
* Inoculate the blood culture bottles immediately after collection. Inject the blood into the aerobic and anaerobic bottles, following the manufacturer’s instructions for the correct order of inoculation.
* Remove the needle from the vein and apply pressure to the venipuncture site with sterile gauze pads until bleeding stops.
* Apply a sterile adhesive bandage to the venipuncture site.
5. **Mixing the Blood Culture Bottles:**
* Gently mix the blood culture bottles by inverting them several times. Avoid vigorous shaking, as this can damage the blood cells.
6. **Labeling and Documentation:**
* Double-check that the blood culture bottles are correctly labeled with all required information.
* Complete the laboratory requisition form with the patient’s information, the source of the specimen (blood culture), the date and time of collection, the number of sets collected, and any relevant clinical information (e.g., fever, chills, suspected infection).
7. **Storage and Transportation:**
* Do not refrigerate the blood culture bottles.
* Transport the blood culture bottles to the laboratory as soon as possible, ideally within 2 hours. Follow the laboratory’s specific transportation guidelines.
8. **Disposal:**
* Dispose of the needle, syringe, and any other contaminated materials in a sharps container.
* Dispose of the gloves and other contaminated materials in a biohazard bag.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
**Laboratory Analysis:**
* The blood culture bottles are incubated in an automated blood culture system that continuously monitors for bacterial growth.
* If bacteria grow in the blood culture, they will be identified and tested for antibiotic susceptibility, including resistance to meticillin.
* If Staphylococcus aureus is isolated and found to be resistant to meticillin, it will be identified as MRSA.
4. Molecular Tests (PCR): Step-by-Step Guide
Polymerase Chain Reaction (PCR) tests are rapid and highly sensitive methods for detecting MRSA DNA. They can be used on nasal swabs, wound swabs, or blood samples.
**Materials Needed:**
* Sterile swab (as appropriate for the sample type: nasal, wound, etc.) or blood collection supplies
* Collection container or tube specified by the PCR test manufacturer
* PCR test kit
* Gloves
* Personal protective equipment (PPE)
* Laboratory requisition form
* Biohazard bag for disposal
**Procedure (General Steps – Specific protocols will vary by kit):**
1. **Preparation:**
* Gather all necessary materials, including the specific PCR test kit and collection supplies.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
* Put on gloves and any other required PPE.
* Label the collection container with the patient’s name, date of birth, date, and time of collection. Ensure it matches the requirements of the PCR test being used.
2. **Sample Collection:**
* Collect the sample according to the type of specimen:
* **Nasal Swab:** Follow the same procedure as described for nasal swab culture.
* **Wound Swab:** Follow the same procedure as described for wound culture.
* **Blood Sample:** Collect blood according to standard venipuncture procedures.
* Follow the PCR test kit’s instructions for the appropriate amount of sample to collect.
3. **Sample Processing (Based on Kit Instructions):**
* Each PCR kit has a specific procedure for processing the sample. This may involve:
* Mixing the sample with reagents provided in the kit.
* Adding the sample to a lysis buffer to break open cells and release DNA.
* Using a specific transfer device to transfer the sample to a reaction tube or cartridge.
* **It is crucial to follow the kit instructions meticulously.**
4. **Running the PCR Test:**
* Most PCR tests for MRSA are automated and require a specialized PCR instrument.
* Place the processed sample into the PCR instrument, following the kit’s instructions.
* Start the PCR run according to the manufacturer’s protocol.
* The PCR instrument will amplify specific MRSA DNA sequences (e.g., the *mecA* gene, which confers meticillin resistance) if they are present in the sample.
5. **Interpreting Results:**
* The PCR instrument will generate a result, indicating whether MRSA DNA was detected or not.
* Positive Result: MRSA DNA was detected, indicating the presence of MRSA.
* Negative Result: MRSA DNA was not detected, indicating the absence of MRSA. However, a negative result does not always rule out MRSA infection, especially if the sample was collected improperly or if the MRSA load is very low.
* The interpretation of results should be done in conjunction with clinical findings and other laboratory data.
6. **Labeling and Documentation:**
* Ensure the sample container is correctly labeled with all required information.
* Complete the laboratory requisition form with the patient’s information, the source of the specimen, the date and time of collection, the type of PCR test performed, and any relevant clinical information.
7. **Storage and Transportation:**
* Store and transport the sample according to the PCR test kit manufacturer’s instructions. Some kits require refrigeration or freezing.
8. **Disposal:**
* Dispose of the gloves and any other contaminated materials in a biohazard bag.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
**Important Considerations for PCR Testing:**
* **Specificity:** PCR tests are highly specific for MRSA DNA. However, false positives can occur due to contamination in the laboratory.
* **Sensitivity:** PCR tests are very sensitive and can detect even small amounts of MRSA DNA. However, false negatives can occur if the sample is not collected properly or if the MRSA load is below the detection limit of the test.
* **Turnaround Time:** PCR tests provide rapid results, often within a few hours, compared to traditional culture methods, which can take 24-48 hours or longer.
* **Cost:** PCR tests are generally more expensive than traditional culture methods.
5. Chromogenic Agar: Step-by-Step Guide
Chromogenic agar is a selective culture medium that contains specific chromogens (color-producing substances) that allow for the visual identification of MRSA colonies. When MRSA grows on the agar, it produces colonies with a characteristic color, making it easier to distinguish from other bacteria.
**Materials Needed:**
* Chromogenic agar plates specifically designed for MRSA detection
* Sterile swab (as appropriate for the sample type)
* Incubator set to 35-37°C
* Gloves
* Personal protective equipment (PPE)
* Laboratory requisition form
* Biohazard bag for disposal
**Procedure:**
1. **Preparation:**
* Gather all necessary materials, including the chromogenic agar plates.
* Ensure the agar plates are not expired and that the packaging is intact.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
* Put on gloves and any other required PPE.
* Label the agar plate with the patient’s name, date of birth, date, and time of inoculation. Ensure it matches the laboratory requisition form.
2. **Sample Collection:**
* Collect the sample according to the type of specimen:
* **Nasal Swab:** Follow the same procedure as described for nasal swab culture.
* **Wound Swab:** Follow the same procedure as described for wound culture.
3. **Inoculation:**
* Open the sterile swab package, being careful not to contaminate the swab.
* Gently roll the swab over the surface of the chromogenic agar plate to evenly distribute the sample. Use a technique that ensures good contact between the swab and the agar surface.
* Avoid applying excessive pressure, which can damage the agar surface.
* If using a liquid sample, such as a broth, apply a small amount to the agar surface and spread it evenly using a sterile loop or spreader.
4. **Incubation:**
* Place the inoculated agar plate in an incubator set to 35-37°C.
* Incubate the plate for 24-48 hours, or as specified by the manufacturer’s instructions.
* Ensure the incubator maintains a stable temperature and humidity level.
5. **Reading the Results:**
* After the incubation period, examine the agar plate for the presence of colonies with the characteristic color associated with MRSA.
* MRSA colonies typically appear as pink, mauve, or rose-colored colonies, depending on the specific chromogenic agar formulation.
* Compare the color of the colonies to the manufacturer’s instructions or a reference chart to confirm the identification of MRSA.
* Note the number and size of the MRSA colonies, as well as any other bacterial growth on the plate.
6. **Confirmation:**
* While chromogenic agar provides a presumptive identification of MRSA, it is important to confirm the identification using additional tests, such as:
* **Latex Agglutination Test:** This test detects the presence of the PBP2a protein, which is associated with meticillin resistance in Staphylococcus aureus.
* **PCR Test:** This test detects the *mecA* gene, which confers meticillin resistance.
* **Antibiotic Susceptibility Testing:** This test determines the susceptibility of the Staphylococcus aureus isolate to various antibiotics, including meticillin.
7. **Labeling and Documentation:**
* Ensure the agar plate is correctly labeled with all required information.
* Complete the laboratory requisition form with the patient’s information, the source of the specimen, the date and time of inoculation, the type of chromogenic agar used, and the results of the plate reading.
8. **Disposal:**
* Dispose of the agar plate and any other contaminated materials in a biohazard bag.
* Wash your hands thoroughly with soap and water or use an alcohol-based hand sanitizer.
**Advantages of Chromogenic Agar:**
* **Ease of Use:** Chromogenic agar is easy to use and requires minimal training.
* **Visual Identification:** The characteristic color of MRSA colonies makes it easy to distinguish from other bacteria.
* **Rapid Results:** Chromogenic agar provides results within 24-48 hours, which is faster than traditional culture methods.
* **Cost-Effective:** Chromogenic agar is relatively inexpensive compared to other MRSA detection methods.
**Limitations of Chromogenic Agar:**
* **Presumptive Identification:** Chromogenic agar provides a presumptive identification of MRSA and requires confirmation with additional tests.
* **Specificity:** Some other bacteria may produce colonies with a similar color to MRSA on chromogenic agar, leading to false positives.
* **Sensitivity:** Chromogenic agar may not detect MRSA if the bacterial load is very low.
Interpreting MRSA Test Results
Interpreting MRSA test results requires careful consideration of the patient’s clinical presentation, risk factors, and the specific test performed.
* **Positive Result:** A positive MRSA test indicates that MRSA is present in the sample. This may indicate:
* **MRSA Colonization:** The individual carries MRSA but does not have an active infection.
* **MRSA Infection:** The individual has an active infection caused by MRSA.
* **Negative Result:** A negative MRSA test indicates that MRSA was not detected in the sample. However, this does not always rule out MRSA colonization or infection, especially if:
* The sample was collected improperly.
* The MRSA load is very low.
* The individual has recently received antibiotics.
In cases where clinical suspicion for MRSA infection remains high despite a negative test result, repeat testing or alternative diagnostic methods may be necessary.
Preventing the Spread of MRSA
Preventing the spread of MRSA is crucial to protect yourself, your family, and your community. Here are some key preventive measures:
* **Hand Hygiene:** Wash your hands frequently with soap and water for at least 20 seconds, or use an alcohol-based hand sanitizer.
* **Avoid Sharing Personal Items:** Do not share towels, razors, clothing, or other personal items.
* **Keep Wounds Clean and Covered:** Clean and cover any cuts, scrapes, or wounds with clean, dry bandages until they are healed.
* **Avoid Contact with Other People’s Wounds:** Avoid touching other people’s wounds or bandages.
* **Shower After Exercise:** Shower immediately after exercise, especially if you use shared equipment.
* **Clean and Disinfect Surfaces:** Regularly clean and disinfect frequently touched surfaces, such as doorknobs, light switches, and countertops.
* **Follow Healthcare Provider Recommendations:** Follow your healthcare provider’s recommendations for preventing the spread of MRSA, including taking antibiotics as prescribed and attending follow-up appointments.
Conclusion
Testing for MRSA is a critical step in diagnosing and managing MRSA infections. Understanding the different testing methods, proper collection techniques, and accurate interpretation of results are essential for healthcare professionals and individuals at risk. By implementing effective preventive measures, we can all play a role in reducing the spread of MRSA and protecting public health. Remember to consult with your healthcare provider for any concerns about MRSA testing or prevention.