Valley fever is diagnosed primarily through blood tests that detect antibodies your immune system produces against the fungus. These blood tests are widely available, require no special preparation like fasting, and are the starting point for nearly all diagnoses. Depending on your symptoms and how your body responds to the infection, your doctor may also use imaging, tissue samples, or spinal fluid analysis to get the full picture.
Blood Tests: The Standard First Step
The CDC recommends starting with a combination of antibody blood tests. These look for two types of immune proteins your body makes in response to the Coccidioides fungus: IgM antibodies, which appear early in the infection, and IgG antibodies, which develop later and can indicate ongoing or more advanced disease.
The most common testing approach pairs an enzyme immunoassay (EIA) with either an immunodiffusion (ID) or complement fixation (CF) test. EIA results come back faster, but ID and CF tests are considered more reliable for confirming the diagnosis. A positive EIA for IgM, when it’s the only positive result, is actually the weakest evidence of infection and can sometimes be a false positive. That’s why doctors typically run more than one type of antibody test at the same time.
No fasting or special preparation is needed before any of these blood draws.
What Antibody Titers Tell You
The complement fixation test doesn’t just tell you positive or negative. It produces a titer, which is a measure of how concentrated the antibodies are in your blood. This number matters because it reflects how active or widespread the infection is.
A titer greater than 1:16 is often associated with the fungus spreading beyond the lungs to bones, skin, or the lining of the brain. Lower titers can still indicate limited spread, such as a single bone or skin lesion. The trend over time matters too: a rising titer is an unfavorable sign suggesting the infection is progressing, while a falling titer typically signals improvement. Your doctor will likely repeat the CF test periodically to track your response to treatment.
The CF test also uses a reactivity scale from 0 to 4+. A score of 4+ is considered significant. A 3+ is meaningful only if the immunodiffusion test is also positive. Scores of 1+ or 2+ are not considered clinically significant on their own.
When Blood Tests Miss the Diagnosis
Antibody tests can come back falsely negative in two situations: early in the infection before your immune system has mounted a detectable response, and in people with weakened immune systems who may not produce enough antibodies to register on the test. If you’re immunocompromised or your symptoms are getting worse despite negative blood work, other diagnostic methods become important.
PCR testing, which detects the fungus’s genetic material directly, has shown strong performance in studies. When used on respiratory samples, one real-time PCR method demonstrated 100% sensitivity and 98.4% specificity compared to culture. For fresh tissue samples, sensitivity was about 93%. PCR is available at reference laboratories, though not every hospital or clinic offers it routinely.
Antigen testing, which looks for proteins shed by the fungus in urine or blood, is another option. It’s typically only positive in patients with extensive or disseminated infections. Recent research suggests antigen testing in spinal fluid may be highly sensitive for detecting valley fever meningitis specifically.
Fungal Culture and Tissue Samples
Growing the fungus in a laboratory culture from a respiratory or tissue sample is the most definitive way to confirm valley fever. If you’re immunosuppressed or your illness is progressing quickly, your doctor may collect respiratory specimens through a bronchoscopy (a thin scope passed into the airways) for culture and microscopic examination.
When the infection has spread beyond the lungs, confirming the diagnosis usually relies on identifying the fungus in tissue from the affected site. Needle aspiration of a skin lesion, bone lesion, or lymph node is a valuable and commonly used approach. The sample is examined under a microscope and sent for culture to look for the characteristic spherical structures the fungus forms inside the body.
The Role of Imaging
Chest X-rays and CT scans don’t diagnose valley fever on their own, but they help establish how much damage the infection has caused in the lungs and guide decisions about further testing.
In acute valley fever, the most common finding on a chest X-ray is a localized area of consolidation (where the lung tissue appears dense and filled with fluid or inflammation). Other findings include nodules, enlarged lymph nodes in the chest, and pleural effusion (fluid around the lungs). Less common patterns include diffuse spots throughout both lungs or miliary disease, which looks like tiny seeds scattered across the lung fields.
Thin-walled cavities are a hallmark of valley fever and show up in roughly 10 to 15% of patients. These cavities form when infected lung tissue breaks down. About half resolve on their own within two years, particularly those smaller than 2.5 cm. Larger cavities over 5 cm are more likely to persist and cause ongoing symptoms. Nodules from valley fever can remain stable for months and typically shrink over time, though slow growth is occasionally seen, which can initially raise concern for cancer.
If you don’t have symptoms suggesting spread beyond the lungs, routine full-body imaging with bone scans, CT, or MRI is generally unnecessary.
Testing for Valley Fever Meningitis
A lumbar puncture (spinal tap) to analyze spinal fluid is not part of a standard valley fever workup. It’s recommended only when specific warning signs suggest the infection has reached the brain or spinal cord: unusual, worsening, or persistent headache, changes in mental clarity, unexplained nausea or vomiting, or new neurological symptoms like weakness or vision changes. Brain or spinal imaging is typically done before the lumbar puncture.
In spinal fluid, doctors look for antibodies against the fungus using the same CF and ID tests used on blood. Antigen detection in spinal fluid is also emerging as a particularly sensitive way to identify valley fever meningitis.
The Skin Test
A skin test for valley fever (brand name Spherusol) does exist and is approved for patients recovering from acute pneumonia caused by the fungus. However, its usefulness is limited. Studies suggest its sensitivity may not exceed 72% compared to current blood-based diagnostics, and many patients with disseminated valley fever test negative. The skin test is better understood as a measure of immune response to past exposure rather than a reliable tool for diagnosing active disease.