The diagnosis of viral meningitis is made by clinical history, physical exam, and several diagnostic tests. Most importantly, cerebrospinal fluid (CSF) is collected via lumbar puncture (also known as spinal tap). This fluid, which normally surrounds the brain and spinal cord, is then analyzed for signs of infection. CSF findings that suggest a viral cause of meningitis include an elevated white blood cell count (usually 10-100 cells/µL) with a lymphocytic predominance in combination with a normal glucose level. Increasingly, cerebrospinal fluid PCR tests have become especially useful for diagnosing viral meningitis, with an estimated sensitivity of 95-100%. Additionally, samples from the stool, urine, blood and throat can also help to identify viral meningitis.

In certain cases, a CT scan of the head should be done before a lumbar puncture such as in those with poor immune function or those with increased intracranial pressure.

Current or previous infection can be detected through a blood test. However, some authors note that such complement-fixation tests are insensitive and should not be used for diagnosis. Dr. Clare A. Dykewicz, "et al." state,

Clinical diagnosis of LCM can be made by the history of prodrome symptoms and by considering the period of time before the onset of meningitis symptoms, typically 15–21 days for LCM.

Pathological diagnosis of congenital infection is performed using either an immunofluorescent antibody (IFA) test or an enzyme immunoassay to detect specific antibody in blood or cerebrospinal fluid. A PCR assay has been recently developed which may be used in the future for prenatal diagnosis; however, the virus is not always present in the blood or CSF when the affected child is born." Diagnoses is subject to methodological shortcomings in regard to specificity and sensitivity of assays used. For this reason, LCMV may be more common than is realized.

Another detection assay is the reverse transcription polymerase chain reaction (RT-PCR) tests which may detect nucleic acids in the blood and cerebrospinal fluid.(CSF) Virus isolation is not used for diagnosis in most cases but it can be isolated from the blood or nasopharyngeal fluid early in the course of the disease, or from CSF in patients with meningitis. LCMV can be grown in a variety of cell lines including BHK21, L and Vero cells, and it may be identified with immuno-fluorescence. A diagnosis can also be made by the intracerebral inoculation of blood or CSF into mice.

It has been proposed that viral meningitis might lead to inflammatory injury of the vertebral artery wall.

The Meningitis Research Foundation is conducting a study to see if new genomic techniques can the speed, accuracy and cost of diagnosing meningitis in children in the UK. The research team will develop a new method to be used for the diagnosis of meningitis, analysing the genetic material of microorganisms found in CSF (cerebrospinal fluid). The new method will first be developed using CSF samples where the microorganism is known, but then will be applied to CSF samples where the microorganism is unknown (estimated at around 40%) to try and identify a cause.

Antigen detection, polymerase chain reaction assay, virus isolation, and serology can be used to identify adenovirus infections. Adenovirus typing is usually accomplished by hemagglutination-inhibition and/or neutralization with type-specific antisera. Since adenovirus can be excreted for prolonged periods, the presence of virus does not necessarily mean it is associated with disease.

Meningitis can be diagnosed after death has occurred. The findings from a post mortem are usually a widespread inflammation of the pia mater and arachnoid layers of the meninges. Neutrophil granulocytes tend to have migrated to the cerebrospinal fluid and the base of the brain, along with cranial nerves and the spinal cord, may be surrounded with pus – as may the meningeal vessels.

As in humans, the sensitivity of testing methods for rodents contributes to the accuracy of diagnosis. LCMV is typically identified through serology. However, in an endemically infected colony, more practical methods include MAP (mouse antibody production) and PCR testing. Another means of diagnosis is introducing a known naïve adult mouse to the suspect rodent colony. The introduced mouse will seroconvert, allowing use of immunofluorescence antibody (IFA), MFIA or ELISA to detect antibodies.

Babies born from mothers with symptoms of Herpes Simplex Virus (HSV) should be tested for viral infection. Liver tests, complete blood count (CBC), cerebrospinal fluid analyses, and chest X-ray should all be completed to diagnose meningitis. Samples should be taken from skin, conjunctiva (eye), mouth and throat, rectum, urine, and the CSF for viral culture and PCR analysis with respect to the sample from CSF.

A lumbar puncture is done by positioning the person, usually lying on the side, applying local anesthetic, and inserting a needle into the dural sac (a sac around the spinal cord) to collect cerebrospinal fluid (CSF). When this has been achieved, the "opening pressure" of the CSF is measured using a manometer. The pressure is normally between 6 and 18 cm water (cmHO); in bacterial meningitis the pressure is usually elevated. In cryptococcal meningitis, intracranial pressure is markedly elevated. The initial appearance of the fluid may prove an indication of the nature of the infection: cloudy CSF indicates higher levels of protein, white and red blood cells and/or bacteria, and therefore may suggest bacterial meningitis.

The CSF sample is examined for presence and types of white blood cells, red blood cells, protein content and glucose level. Gram staining of the sample may demonstrate bacteria in bacterial meningitis, but absence of bacteria does not exclude bacterial meningitis as they are only seen in 60% of cases; this figure is reduced by a further 20% if antibiotics were administered before the sample was taken. Gram staining is also less reliable in particular infections such as listeriosis. Microbiological culture of the sample is more sensitive (it identifies the organism in 70–85% of cases) but results can take up to 48 hours to become available. The type of white blood cell predominantly present (see table) indicates whether meningitis is bacterial (usually neutrophil-predominant) or viral (usually lymphocyte-predominant), although at the beginning of the disease this is not always a reliable indicator. Less commonly, eosinophils predominate, suggesting parasitic or fungal etiology, among others.

The concentration of glucose in CSF is normally above 40% of that in blood. In bacterial meningitis it is typically lower; the CSF glucose level is therefore divided by the blood glucose (CSF glucose to serum glucose ratio). A ratio ≤0.4 is indicative of bacterial meningitis; in the newborn, glucose levels in CSF are normally higher, and a ratio below 0.6 (60%) is therefore considered abnormal. High levels of lactate in CSF indicate a higher likelihood of bacterial meningitis, as does a higher white blood cell count. If lactate levels are less than 35 mg/dl and the person has not previously received antibiotics then this may rule out bacterial meningitis.

Various other specialized tests may be used to distinguish between different types of meningitis. A latex agglutination test may be positive in meningitis caused by "Streptococcus pneumoniae", "Neisseria meningitidis", "Haemophilus influenzae", "Escherichia coli" and "group B streptococci"; its routine use is not encouraged as it rarely leads to changes in treatment, but it may be used if other tests are not diagnostic. Similarly, the limulus lysate test may be positive in meningitis caused by Gram-negative bacteria, but it is of limited use unless other tests have been unhelpful. Polymerase chain reaction (PCR) is a technique used to amplify small traces of bacterial DNA in order to detect the presence of bacterial or viral DNA in cerebrospinal fluid; it is a highly sensitive and specific test since only trace amounts of the infecting agent's DNA is required. It may identify bacteria in bacterial meningitis and may assist in distinguishing the various causes of viral meningitis (enterovirus, herpes simplex virus 2 and mumps in those not vaccinated for this). Serology (identification of antibodies to viruses) may be useful in viral meningitis. If tuberculous meningitis is suspected, the sample is processed for Ziehl-Neelsen stain, which has a low sensitivity, and tuberculosis culture, which takes a long time to process; PCR is being used increasingly. Diagnosis of cryptococcal meningitis can be made at low cost using an India ink stain of the CSF; however, testing for cryptococcal antigen in blood or CSF is more sensitive, particularly in people with AIDS.

A diagnostic and therapeutic difficulty is "partially treated meningitis", where there are meningitis symptoms after receiving antibiotics (such as for presumptive sinusitis). When this happens, CSF findings may resemble those of viral meningitis, but antibiotic treatment may need to be continued until there is definitive positive evidence of a viral cause (e.g. a positive enterovirus PCR).

A lumbar puncture (LP) is necessary to diagnose meningitis. Cerebrospinal fluid (CSF) culture is the most important study for the diagnosis of neonatal bacterial meningitis because clinical signs are non-specific and unreliable. Blood cultures may be negative in 15-55% of cases, deeming it unreliable as well. However, a CSF/blood glucose ratio below two-thirds has a strong relationship to bacterial meningitis. A LP should be done in all neonates with suspected meningitis, with suspected or proven sepsis (whole body inflammation) and should be considered in all neonates in whom sepsis is a possibility. The role of the LP in neonates who are healthy appearing but have maternal risk factors for sepsis is more controversial; the yield of the LP in these patients may be low.

Early-onset is deemed when infection is within one week of birth. Late-onset is deemed after the first week.

People should only be diagnosed with encephalitis if they have a decreased or altered level of consciousness, lethargy, or personality change for at least twenty-four hours without any other explainable cause. Diagnosing encephalitis is done via a variety of tests:

- Brain scan, done by MRI, can determine inflammation and differentiate from other possible causes.

- EEG, in monitoring brain activity, encephalitis will produce abnormal signal.

- Lumbar puncture (spinal tap), this helps determine via a test using the cerebral-spinal fluid, obtained from the lumbar region.

- Blood test

- Urine analysis

- Polymerase chain reaction (PCR) testing of the cerebrospinal fluid, to detect the presence of viral DNA which is a sign of viral encephalitis.

Vaccination is available against tick-borne and Japanese encephalitis and should be considered for at-risk individuals. Post-infectious encephalomyelitis complicating smallpox vaccination is avoidable, for all intents and purposes, as smallpox is nearly eradicated. Contraindication to Pertussis immunization should be observed in patients with encephalitis.

If suspected, fungal meningitis is diagnosed by testing blood and CSF samples for pathogens. Identifying the specific pathogen is necessary to determine the proper course of treatment and the prognosis. Measurement of opening pressure, cell count with differential, glucose and protein concentrations, Gram's stain, India ink, and culture tests should be preformed on CSF samples when fungal meningitis is suspected.

The best prevention against viral pneumonia is vaccination against influenza, adenovirus, chickenpox, herpes zoster, measles, and rubella.

Because the risk of meningococcal disease is increased among USA's military recruits, all military recruits routinely receive primary immunization against the disease.

Safe and effective adenovirus vaccines were developed for adenovirus serotypes 4 and 7, but were available only for preventing ARD among US military recruits, and production stopped in 1996. Strict attention to good infection-control practices is effective for stopping transmission in hospitals of adenovirus-associated disease, such as epidemic keratoconjunctivitis. Maintaining adequate levels of chlorination is necessary for preventing swimming pool-associated outbreaks of adenovirus conjunctivitis.

Diagnosis starts by examining the patient's symptoms. Symptoms can vary. Symptoms can include headache, sensitivity to light, neck stiffness, nausea, and vomiting. In some patients, fever is absent. Neurological examination and MRI can be normal.

Mollaret's meningitis is suspected based on symptoms, and can be confirmed by HSV 1 or HSV 2 on PCR of Cerebrospinal fluid (CSF), although not all cases test positive on PCR. PCR is performed on spinal fluid or blood, however, the viruses do not need to enter the spinal fluid or blood to spread within the body: they can spread by moving through the axons and dendrites of the nerves.

During the first 24 h of the disease the spinal fluid will show predominant polymorphonuclear neutrophils and large cells that have been called endothelial (Mollaret’s) cells.

A study performed on patients who had diffuse symptoms, such as persistent or intermittent headaches, concluded that although PCR is a highly sensitive method for detection, it may not always be sensitive enough for identification of viral DNA in CSF, due to the fact that viral shedding from latent infection may be very low. The concentration of viruses in CSF during subclinical infection might be very low.

Investigations include blood tests (electrolytes, liver and kidney function, inflammatory markers and a complete blood count) and usually X-ray examination of the chest. The most important test in identifying or ruling out meningitis is analysis of the cerebrospinal fluid (fluid that envelops the brain and the spinal cord) through lumbar puncture (LP). However, if the patient is at risk for a cerebral mass lesion or elevated intracranial pressure (recent head injury, a known immune system problem, localizing neurological signs, or evidence on examination of a raised ICP), a lumbar puncture may be contraindicated because of the possibility of fatal brain herniation. In such cases, a CT or MRI scan is generally performed prior to the lumbar puncture to exclude this possibility. Otherwise, the CT or MRI should be performed after the LP, with MRI preferred over CT due to its superiority in demonstrating areas of cerebral edema, ischemia, and meningeal inflammation.

During the lumbar puncture procedure, the opening pressure is measured. A pressure of over 180 mm HO is suggestive of bacterial meningitis.

It is likely that Mollaret meningitis is underrecognized by physicians, and improved recognition may limit unwarranted antibiotic use and shorten or eliminate unnecessary hospital admission.

PCR testing has advanced the state of the art in research, but PCR can be negative in individuals with Mollaret's, even during episodes with severe symptoms. For example, Kojima et al. published a case study for an individual who was hospitalized repeatedly, and who had clinical symptoms including genital herpes lesions. However, the patient was sometimes negative for HSV-2 by PCR, even though his meningitis symptoms were severe. Treatment with acyclovir was successful, indicating that a herpes virus was the cause of his symptoms.

It is recommended that primary immunization against meningococcal disease with meningitis A,C,Y and W-135 vaccines for all young adolescents at 11–12 years of age and all unvaccinated older adolescents at 15 years of age. Although conjugate vaccines are the preferred meningococcal vaccine in adolescents 11 years of age or older, polysaccharide vaccines are an acceptable alternative if the conjugated vaccine is unavailable.

A number of various diseases may present with symptoms similar to those caused by a clinical West Nile virus infection. Those causing neuroinvasive disease symptoms include the enterovirus infection and bacterial meningitis. Accounting for differential diagnoses is a crucial step in the definitive diagnosis of WNV infection. Consideration of a differential diagnosis is required when a patient presents with unexplained febrile illness, extreme headache, encephalitis or meningitis. Diagnostic and serologic laboratory testing using polymerase chain reaction (PCR) testing and viral culture of CSF to identify the specific pathogen causing the symptoms, is the only currently available means of differentiating between causes of encephalitis and meningitis.

Development of new therapies has been hindered by the lack of appropriate animal model systems for some important viruses and also because of the difficulty in conducting human clinical trials for diseases that are rare. Nonetheless, numerous innovative approaches to antiviral therapy are available including candidate thiazolide and purazinecarboxamide derivatives with potential broad-spectrum antiviral efficacy. New herpes virus drugs include viral helicase-primase and terminase inhibitors. A promising new area of research involves therapies based on enhanced understanding of host antiviral immune responses.

Preliminary diagnosis is often based on the patient's clinical symptoms, places and dates of travel (if patient is from a nonendemic country or area), activities, and epidemiologic history of the location where infection occurred. A recent history of mosquito bites and an acute febrile illness associated with neurologic signs and symptoms should cause clinical suspicion of WNV.

Diagnosis of West Nile virus infections is generally accomplished by serologic testing of blood serum or cerebrospinal fluid (CSF), which is obtained via a lumbar puncture. Initial screening could be done using the ELISA technique detecting immunoglobulins in the sera of the tested individuals.

Typical findings of WNV infection include lymphocytic pleocytosis, elevated protein level, reference glucose and lactic acid levels, and no erythrocytes.

Definitive diagnosis of WNV is obtained through detection of virus-specific antibody IgM and neutralizing antibodies. Cases of West Nile virus meningitis and encephalitis that have been serologically confirmed produce similar degrees of CSF pleocytosis and are often associated with substantial CSF neutrophilia.

Specimens collected within eight days following onset of illness may not test positive for West Nile IgM, and testing should be repeated. A positive test for West Nile IgG in the absence of a positive West Nile IgM is indicative of a previous flavavirus infection and is not by itself evidence of an acute West Nile virus infection.

If cases of suspected West Nile virus infection, sera should be collected on both the acute and

convalescent phases of the illness. Convalescent specimens should be collected 2–3 weeks after acute specimens.

It is common in serologic testing for cross-reactions to occur among flaviviruses such as dengue virus (DENV) and tick-borne encephalitis virus; this necessitates caution when evaluating serologic results of flaviviral infections.

Four FDA-cleared WNV IgM ELISA kits are commercially available from different manufacturers in the U.S., each of these kits is indicated for use on serum to aid in the presumptive laboratory diagnosis of WNV infection in patients with clinical symptoms of meningitis or encephalitis. Positive WNV test results obtained via use of these kits should be confirmed by additional testing at a state health department laboratory or CDC.

In fatal cases, nucleic acid amplification, histopathology with immunohistochemistry, and virus culture of autopsy tissues can also be useful. Only a few state laboratories or other specialized laboratories, including those at CDC, are capable of doing this specialized testing.

Diagnosis of TB meningitis is made by analysing cerebrospinal fluid collected by lumbar puncture. When collecting CSF for suspected TB meningitis, a minimum of 1ml of fluid should be taken (preferably 5 to 10ml). The CSF usually has a high protein, low glucose and a raised number of lymphocytes. Acid-fast bacilli are sometimes seen on a CSF smear, but more commonly, "M. tuberculosis" is grown in culture. A spiderweb clot in the collected CSF is characteristic of TB meningitis, but is a rare finding. ELISPOT testing is not useful for the diagnosis of acute TB meningitis and is often false negative, but may paradoxically become positive after treatment has started, which helps to confirm the diagnosis.

This is a group of tests that use polymerase chain reaction (PCR) to detect mycobacterial nucleic acid. These test vary in which nucleic acid sequence they detect and vary in their accuracy. The two most common commercially available tests are the amplified mycobacterium tuberculosis direct test (MTD, Gen-Probe) and Amplicor. In 2007, review concluded that for diagnosing tuberculous meningitis "Individually, the AMTD test appears to perform the best (sensitivity 74% and specificity 98%)", they found the pooled prevalence of TB meningitis to be 29%.

Prophylactic vaccination is available against poliomyelitis, measles, Japanese encephalitis, and rabies. Hyper immune immunoglobulin has been used for prophylaxis of measles, herpes zoster virus, HSV-2, vaccine, rabies, and some other infections in high-risk groups.

Animal pathogens exist as facultative parasites. They are an exceptionally rare cause of meningoencephalitis.

clinical diagnosis include recurrent or recent herpes infection fever, headache, mental symptom, convulsion, disturbance of consciousness, focal signs.

CSF ,EEG, CT, MRI are responsive to specific antivirus agent.

Definite diagnosis – besides the above, the followings are needed

CSF: HSV－antigen,HSV－Antibody, brain biopsy or pathology: Cowdry in intranuclear

CSF: the DNA of the HSV(PCR)

cerebral tissue or specimen of the CSF:HSV

except other viral encephalitis

Prognosis depends on the pathogen responsible for the infection and risk group. Overall mortality for "Candida" meningitis is 10-20%, 31% for patients with HIV, and 11% in neurosurgical cases (when treated). Prognosis for "Aspergillus" and coccidioidal infections is poor.