The FebriDx test is a rapid point-of-care test that analyzes a small fingerstick blood sample for two host-response biomarkers: Myxovirus resistance protein A (MxA) and C-reactive protein (CRP). Together, these biomarkers help healthcare professionals differentiate bacterial from non-bacterial causes of acute respiratory infection.
The test provides results in approximately 10 minutes, making it useful when a clinician needs additional information during an acute respiratory infection assessment. FebriDx results should always be considered alongside the patient’s symptoms, clinical examination, and other available diagnostic information.
How Does the FebriDx Test Work?
FebriDx uses two biomarkers that provide information about the body’s immune response:
- MxA: MxA levels can increase during an acute viral infection.
- CRP: CRP is a non-specific marker of inflammation and can increase during bacterial infections as well as some viral infections.
Neither marker should be interpreted on its own. The FebriDx test combines the two results to help clinicians assess whether a patient’s immune response is more consistent with a bacterial or non-bacterial respiratory infection.
Step-by-Step FebriDx Test Instructions
FebriDx is designed for professional healthcare use. The manufacturer’s instructions should be followed carefully when performing the test.
1. Prepare the Test
Remove the FebriDx device from its sealed pouch only when you are ready to perform the test. Check that the test has not expired and that the packaging is intact.
Standard precautions should be followed when collecting and handling the blood specimen.
2. Prepare the Finger
Clean the fingertip with an alcohol pad and allow it to air dry.
Remove the protective tab from the built-in lancet and firmly press the lancet against the fingertip to obtain a blood sample. The first drop of blood should be wiped away with gauze.
3. Collect the Blood Sample
Gently massage the finger to encourage blood flow and produce a hanging drop of blood.
Position the blood collection tube at approximately a 45-degree angle beneath the finger and allow the blood to enter the collection tube. The device uses capillary action to collect the required sample.
4. Transfer the Blood to the Test
Once the blood has been collected, follow the device’s sequential steps to transfer the sample to the test strip.
The blood transfer confirmation window should change from white to pink/red, indicating that the sample has been transferred correctly. If the confirmation window does not change as expected, the test should be discarded and the patient retested using a new device.
5. Activate the Test
Release the buffer according to the manufacturer’s instructions. The buffer allows the collected sample to move through the test strip and enables detection of the MxA and CRP biomarkers.
6. Wait for the Result
Place the test on a suitable flat surface and allow the test to develop.
Read the result at 10 minutes. If the blood has not completely cleared at 10 minutes, wait for the background to clear before interpreting the result. The manufacturer’s quick-reference instructions state that results should not be interpreted after one hour.
How to Interpret FebriDx Results
One of the most important concepts when interpreting FebriDx is understanding the distinction between bacterial and non-bacterial results. The FebriDx result window contains lines associated with CRP, MxA, and the control.
A blue control line must appear for the test to be considered valid. If the blue control line is absent, the result is invalid and the patient should be retested using a new FebriDx device.
Bacterial Result
A black result line without a red MxA result line, together with the required blue control line, indicates a result consistent with a bacterial infection.
This pattern reflects an elevated CRP response without an elevated MxA response.
Viral or Non-Bacterial Result
A red MxA result line, with or without a CRP result line, indicates a result consistent with a viral infection or other non-bacterial etiology.
MxA is associated with the body’s immune response to acute viral infection. However, the test does not identify a specific virus.
If both MxA and CRP result lines are present, a viral infection is indicated, but a bacterial co-infection cannot be excluded.
Negative Result
If there is no CRP or MxA result line, while the blue control line is present, the test is considered negative.
A negative result means the test did not detect the relevant biomarker pattern at the test’s decision thresholds. However, a negative FebriDx result does not rule out a respiratory infection and should not be used by itself to make diagnosis or treatment decisions.
Invalid Result
If the blue control line does not appear, the test is invalid.
Do not interpret the result as positive or negative. A new FebriDx test should be performed according to the manufacturer’s instructions.
What If the FebriDx Result Line Is Faint?
A faint result line should not automatically be considered negative.
According to the manufacturer’s guidance, even if a result line is faint, does not extend completely across the width of the result window, or appears uneven in color, it should be interpreted as positive when the test is otherwise valid.
FebriDx Result Interpretation at a Glance
| Result Pattern | General Interpretation |
|---|---|
| Black CRP line + blue control line, no red MxA line | Consistent with bacterial infection |
| Red MxA line + blue control line | Consistent with viral/non-bacterial etiology |
| Red MxA + CRP lines + blue control line | Viral result; bacterial co-infection cannot be excluded |
| No CRP or MxA result line + blue control line | Negative |
| No blue control line | Invalid test |
Important Considerations When Interpreting FebriDx
FebriDx is intended to support clinical assessment, not replace it. The test does not identify a specific bacterial or viral pathogen, and results should be interpreted together with the patient’s symptoms, examination findings, and other relevant clinical or diagnostic information.
It is also important to follow the manufacturer’s instructions for specimen collection, test timing, storage, and interpretation. Incorrect sample collection or an invalid control result can affect the reliability of the test.
Conclusion
The FebriDx test provides a rapid way to assess two important host-response biomarkers, MxA and CRP, from a fingerstick blood sample. By combining these biomarkers, the test can help healthcare professionals distinguish a bacterial pattern from a viral or other non-bacterial pattern in patients being evaluated for acute respiratory infection.
For accurate interpretation, the test should be performed according to the manufacturer’s instructions, read at the appropriate time, and evaluated together with the patient’s overall clinical presentation.
