From the Guidelines
If influenza and Covid-19 tests are negative, common flu labs to consider include a complete blood count (CBC) to evaluate for signs of infection or inflammation, and a respiratory viral panel to detect other potential viral causes of symptoms, such as respiratory syncytial virus (RSV), adenovirus, and human metapneumovirus 1.
Key Laboratory Tests
- A blood culture and a chest X-ray may be ordered to rule out bacterial pneumonia or other complications 1.
- Liver function tests, such as alanine aminotransferase, aspartate aminotransferase, and lactate dehydrogenase, may be considered to evaluate for potential liver involvement 1.
- Procalcitonin levels may be measured to assess for bacterial infection 1.
Additional Considerations
- A nasopharyngeal swab for pertussis (whooping cough) may also be considered in some cases 1.
- Medications such as oseltamivir or zanamivir may still be prescribed if the clinical presentation is consistent with influenza, despite a negative test result, due to the limitations of testing sensitivity 1.
- Antibiotics such as azithromycin or amoxicillin-clavulanate may be prescribed if a bacterial infection is suspected 1. However, it's crucial to consult with a healthcare professional for personalized evaluation and treatment, as the management of flu-like symptoms with negative influenza and COVID-19 tests can be complex and requires careful consideration of various factors, including the patient's medical history, physical examination, and laboratory results 1.
From the Research
Laboratory Tests for Flu-like Symptoms
When patients present with flu-like symptoms but test negative for influenza and COVID-19, several laboratory tests can be conducted to determine the underlying cause of their symptoms. These tests include:
- Complete Blood Count (CBC) to evaluate white blood cell count, platelet count, and differential count 2, 3, 4
- C-reactive protein (CRP) test to assess inflammation levels 2
- Blood chemistry tests to evaluate liver and kidney function
- Chest X-ray to assess lung abnormalities, although this may not be effective in distinguishing between COVID-19 and influenza 5
Blood Parameters
Studies have shown that patients with COVID-19 and influenza have distinct blood parameter profiles. For example:
- Patients with COVID-19 tend to have lower lymphocyte counts and higher neutrophil counts compared to patients with influenza 3, 4
- Patients with influenza tend to have higher white blood cell counts and higher platelet counts compared to patients with COVID-19 3, 4
- Monocyte count and percentage of basophils can be used to differentiate between COVID-19 and influenza 3
Diagnostic Challenges
Distinguishing between COVID-19 and influenza can be challenging, especially during the early stages of the disease. A combo rapid antigen test, such as QuickNavi-Flu+COVID19 Ag, can detect both SARS-CoV-2 and influenza viruses simultaneously, but its sensitivity and specificity may vary depending on the sample type and patient symptoms 6. Therefore, a comprehensive diagnostic approach, including clinical evaluation, laboratory tests, and imaging studies, is necessary to determine the underlying cause of flu-like symptoms in patients with negative influenza and COVID-19 tests.