Blood tests are commonly used to help diagnose many types of infections, including some that affect the brain and central nervous system. Detecting a brain or nervous system infection through blood work depends on the type of infection, its location, severity, and other factors.
Quick Answers
– Certain brain infections, like meningitis, often show abnormalities in blood tests.
– Blood cultures can sometimes detect bacteria causing brain abscesses or meningitis.
– Viral brain infections like herpes encephalitis may not always be detectable in the blood.
– Fungal and parasitic brain infections can show up through blood antibody tests in some cases.
– Prion diseases like Creutzfeldt-Jakob disease currently lack blood-based diagnostic tests.
How Blood Tests Help Diagnose Brain Infections
Blood tests play an important role in identifying and diagnosing many types of neurological infections. Some key ways blood work contributes to detecting brain infections include:
- Complete blood count (CBC) – Elevated white blood cell count often indicates infection or inflammation in the body.
- Blood cultures – Can detect bacteria and fungi in the blood that may be causing a brain abscess or meningitis.
- Chemistry panel – Abnormal electrolytes, kidney function, and enzymes may point to an infectious process.
- Inflammatory markers – Tests like C-reactive protein and erythrocyte sedimentation rate can confirm the presence of infection.
- Microbiology – Identification of specific viruses, bacteria, or parasites and associated antibodies.
- Coagulation tests – Abnormal clotting seen with some severe infections.
- Molecular techniques – PCR and DNA testing able to identify traces of pathogens.
Results from blood work provide supporting evidence for central nervous system infections and can help pinpoint the particular pathogen involved. However, blood tests cannot directly confirm the diagnosis on their own.
Brain Infections Potentially Detectable Through Blood Work
Many common brain infections will produce some type of anomaly in blood testing that supports the diagnosis. But not all neurological infections yield positive blood findings.
Bacterial Infections
Bacterial meningitis – Meningitis causes an inflammation of the membranes (meninges) surrounding the brain and spinal cord. Bacterial varieties are among the most common causes of meningitis. Blood tests often show high white blood cell count, positive blood cultures, and elevated procalcitonin and CRP levels with bacterial meningitis.
Brain abscess – Pockets of pus and infection can form inside the brain tissue. Blood cultures may identify the bacteria responsible, often species like Staphylococcus, Streptococcus, and E. coli.
Neurosyphilis – Late stage syphilis infections can invade the central nervous system. The bacteria that cause syphilis can be detected through blood tests.
Lyme neuroborreliosis – Advanced Lyme disease caused by the bacteria Borrelia burgdorferi can affect the nervous system. Blood tests look for Lyme antibodies and may use PCR testing.
Neuroleptospirosis – Bacterial infection from Leptospira interrogans that can lead to inflammation of the brain. Blood samples can identify Leptospira antibodies.
Viral Infections
Encephalitis – Brain inflammation causes by viruses such as herpes simplex, varicella zoster, or rabies. Blood PCR tests may detect traces of viral genetic material in some cases.
Meningitis – Viral meningitis due to enteroviruses, herpes, mumps virus, HIV, or others. Blood antibody tests can sometimes confirm the specific virus present.
Progressive multifocal leukoencephalopathy (PML) – Caused by the JC polyomavirus in immunocompromised people. Blood tests look for JC virus antibodies.
Subacute sclerosing panencephalitis (SSPE) – Rare delayed complication of measles virus infection. Measles antibodies are elevated in the blood.
Fungal Infections
Cryptococcal meningitis – The fungus Cryptococcus neoformans is one of the most common causes of fungal meningitis, often seen in those with HIV/AIDS. Blood cultures or antibody tests can detect Cryptococcus.
Coccidioidal meningitis – Fungal infection caused by Coccidioides immitis found in certain geographic areas. Blood tests look for Coccidioides antibodies.
Candida meningitis – The yeast Candida albicans only rarely spreads to the meninges but can be detected through blood cultures when it does.
Parasitic Infections
Neurocysticercosis – Brain infection caused by the pork tapeworm Taenia solium. Blood tests can check for antibodies against the parasite.
Primary amebic meningoencephalitis (PAM) – Brain infection caused by the ameba Naegleria fowleri. Specific blood antibody testing can confirm recent infection.
Toxoplasma encephalitis – Reactivation of dormant Toxoplasma gondii infection in immunocompromised individuals. Toxoplasma IgG antibodies are present in the blood.
Prion Diseases
Creutzfeldt-Jakob disease (CJD) – Rapidly progressive and fatal brain disorder caused by prions. No reliable blood test currently exists for CJD.
Variant CJD – Linked to bovine spongiform encephalopathy (mad cow disease). Blood and tonsil biopsies can be done for vCJD but lack accuracy.
Autoimmune Encephalitis
Some forms of encephalitis are caused by the immune system mistakenly attacking brain tissue. Certain autoimmune forms like anti-NMDA receptor encephalitis can be associated with antibodies detectable in the blood through testing.
Limitations of Blood Testing for Brain Infections
Despite the ability to pick up signs of many central nervous system infections through blood work, important limitations exist:
- Not 100% sensitive – Negative blood tests do not definitively rule out a neurological infection.
- Do not pinpoint location – Blood cannot specify if infection is in the brain, meninges, or spinal cord.
- Often nonspecific – Elevated white blood cells or inflammatory markers indicate infection but not where.
- Limited antibody response – Certain viruses may not trigger strong detectable antibody response.
- Prior antibiotics – Treatment before blood cultures are drawn can cause false negative results.
- Timing issues – Early stages of infection may not yet show abnormalities.
Because of these limitations, blood test results need to be interpreted within the full clinical context and should not be solely relied upon to rule out a neurological infection. Neuroimaging, lumbar puncture, and other testing is often needed to complement blood work.
Brain Infections That May Not Show in Blood
While many brain infections can produce clues in blood test results, some do not, including:
Viral Encephalitis
Certain types of viral brain infections, including encephalitis from herpes simplex virus, may not always be detectable through blood tests. PCR testing can sometimes identify traces of viral genetic material in the bloodstream, but results are inconsistent.
Neurosyphilis
While a syphilis infection generally produces antibodies that remain for life, later stage neurosyphilis does not always generate high enough antibody titers to be picked up through standard blood tests.
Prion diseases
Transmissible spongiform encephalopathies like Creutzfeldt-Jakob disease (CJD) are caused by abnormal prion proteins accumulating in the brain. There are currently no standard, validated blood tests to diagnose CJD.
Autoimmune encephalitis
In some forms of autoimmune encephalitis, like anti-NMDA receptor encephalitis, antibodies are detectable. But other types may not involve measurable antibodies, limiting blood testing.
Fungal infections
Fungal brain abscesses and meningitis only appear through blood cultures in a minority of cases. Fungal elements and antigens can be difficult to detect even when present.
Parasitic infections
Certain parasitic infections affecting the central nervous system, such as toxoplasmosis, may not always trigger a measurable antibody response in the blood.
Role of Lumbar Puncture in Diagnosis
In addition to blood work, analysis of cerebrospinal fluid (CSF) through a lumbar puncture is critical in diagnosing many suspected brain infections. CSF testing looks for:
- White blood cells – Elevated white cell count indicates inflammation/infection.
- Glucose levels – Low glucose common with bacterial infections.
- Protein – Increased protein suggests infection, inflammation, or barrier disruption.
- Microbiology – Bacterial and fungal cultures, parasite testing, PCR for viruses.
- Flow cytometry – Cells present in CSF provide diagnostic clues.
CSF findings along with blood test results allow for a comprehensive assessment of potential central nervous system infections. In some circumstances, positive CSF results may be present even when blood tests are normal.
Conclusion
Blood testing plays an important but limited role in diagnosing brain infections. It provides supporting evidence that must be considered along with the patient’s symptoms, medical history, imaging results, lumbar puncture findings, and other factors.
Certain types of neurological infections routinely produce abnormalities on blood work that contributes to the diagnosis. However, blood tests cannot isolate the specific infected area in the brain or nervous system. And some brain infections fail to generate any positive blood findings at all.
Ultimately, blood work alone cannot definitively rule in or exclude a suspected brain infection. But when interpreted carefully in the full clinical context, blood test results can often yield valuable clues pointing to an infectious process requiring further diagnostic testing and prompt treatment.