You’ve been dealing with unusual infections or symptoms that just don’t seem to add up, and your doctor suspects it might be something rare — like CARD9 deficiency. What now? It can be confusing and even scary when getting a diagnosis for a rare immune condition. But don’t worry, we’re here to break it down. In this article, you’ll learn about what CARD9 deficiency actually is, why genetic testing is so important for this, and how doctors confirm the diagnosis with confidence, all in simple and clear terms.

Answer the question

You don’t usually get diagnosed with CARD9 deficiency right away. It often starts when doctors run genetic tests to check for mutations in both copies of your CARD9 gene. If they find something suspicious, they’ll usually want to double-check — maybe by running another sequencing method, just to be sure it’s not an error. 

CARD9 deficiency is diagnosed through genetic testing of the CARD9 gene, commonly by DNA sequencing to look for disease-causing mutations on both copies of the gene¹. The presence of such mutations confirms the diagnosis by checking the genetic findings (e.g., by a second sequencing test) and running specific lab studies to prove the immune defect². These studies could test for a lack of CARD9 protein on the patient’s white blood cells or indicate an abnormal response by the patient’s immune cells to fungal organisms³. Collectively, these procedures build a firm diagnosis of CARD9 deficiency.

Read on to understand what CARD9 deficiency is, how it is diagnosed, and why early genetic testing is recommended, where it can lead to better outcomes and targeted treatment options.

Introduction

Deficiency of Caspase recruitment domain-containing protein 9 (CARD9) is a rare genetic immune disorder with heightened susceptibility to fungal infections. CARD9 deficiency is caused by mutations of the gene coding for the CARD9 protein, responsible for the instruction on how to make a protein for anti-fungal immunity, resulting in defective immune reaction to fungal pathogens.⁴ Without functional CARD9, the body cannot resist certain fungal pathogens, including the species of Candida, Aspergillus, and Phialophora, resulting in life-threatening fungal infections of the central nervous system, lungs, bones, or skin.⁵

How common or rare is it? CARD9 deficiency is extremely rare – globally, there are barely more than several dozen recognised cases reported in the medical literature to date. For example, through 2018, there were 58 recognised patients from 39 families reported on multiple continents. Over 20 different mutations of the CARD9 gene can result in the disease (missense, nonsense, frameshift alterations, etc.), but they all fundamentally abolish CARD9 function.⁶

This is an autosomal recessive condition, so the individual inherits two defective copies of the CARD9 gene, inherited from each carrier parent, to produce symptoms. (Individuals with one defective copy of the gene become carriers but do not exhibit immunodeficiency symptoms). For some communities or racial groups, there are specific CARD9 mutations that recur because of a founder effect (shared origin ancestry), but the condition, as a whole, can be found within people of every origin. Due to the rarity of CARD9 deficiency, it remains undiagnosed until the individual contracts a severe infection. Without the correct advice, these infections prove fatal. Research acknowledges that approximately 15–20% of identified patients succumbed to the fungal illness complications. Diagnosis, therefore, has to take place early to control infections and enhance the outcome.⁷

Clinical signs and testing for CARD9 deficiency

When should CARD9 deficiency be suspected?

CARD9 deficiency is an exceedingly rare genetic disease that impairs the host's ability to fight fungal infections. The doctors will take this disease into account among otherwise healthy individuals or those with serious or new fungal infections.

CARD9 deficiency is a consideration if an individual has:

• Recurrent or deep fungal infections (e.g., brain, liver, or skin infections)
• Infections that don’t respond well to typical antifungal treatments
• Family history of fungal infections or confirmed CARD9 deficiency
• Fungal infections in rare locations, such as the brain or the lymph nodes

Hints from primary screening

Before they order genetic tests, physicians typically perform:

• Blood tests (white cell counts, inflammation markers)
• Fungal cultures or biopsies from the infected areas
• Screening for other immune conditions (e.g. HIV, primary immunodeficiencies)

This helps rule out more common causes and strengthens the case for CARD9-specific testing.

Red flags that raise suspicion

Several features hint at CARD9 deficiency more than others:

Invasive candida infections in the brain or organs

Conditions such as spontaneous Candida meningitis or liver abscesses, particularly among individuals without other immune issues, are big red flags.

Deep or recalcitrant skin fungal infections

Common skin fungi (such as ringworm) become widespread and deep, leading to severe infections of the scalp or skin that fail to respond to usual therapy, a CARD9 deficiency.

Mycotoxins generated by rare

Infections caused by environmental moulds like Exophiala (a black mould) are uncommon in healthy people. Their presence can signal a hidden immune problem.

Invasive aspergillosis without identified risk factors

While generally related to chemotherapy or HIV, there are CARD9-deficient individuals who develop aspergillosis without these exposures.

Chronic oral or skin thrush

Occasionally, chronic mucocutaneous candidiasis (CMC) will be an early indicator. Although other causes of CMC can occur, extremely severe or persistent symptoms could suggest CARD9.

It is worth mentioning, however, that infections with bacteria or viruses are not characteristic in CARD9 deficiency – the problem is virtually always fungal. Age of onset is highly variable, so physicians need to be on the alert.

How is CARD9 deficiency diagnosed?

Genetic testing

The gold standard for diagnosis is genetic testing to detect mutations in the CARD9 gene. This can be done using.

Single-gene testing

When the level of suspicion is high (e.g., childhood brain Candida infection), doctors will sequence the CARD9 gene by itself.

Gene panel testing

More commonly utilised is the primary immunodeficiency gene panel screen, where multiple genes for fungal susceptibility, including the CARD9 gene, are screened.

Whole exome or genome sequencing

If no mutations come up but there is suspicion, physicians can order whole exome sequencing (WES) or whole genome sequencing (WGS) to identify rare variants or new causes.

What confirms the diagnosis?

It is characterised by the identification of both alleles of the CARD9 gene to possess disease-causing mutations. These are usually:

• Loss-of-function mutations (e.g. premature stop codons or frameshifts
• Compound heterozygous mutations (different mutations inherited from each parent)

Having a single mutation doesn't prove the diagnosis; additional testing would be required in this instance to investigate other causes.

Confirming the diagnosis beyond DNA

Once genetic testing suggests CARD9 deficiency, confirmatory steps help solidify the diagnosis:

Rechecking the mutation

Labs will typically utilise Sanger sequencing, an extremely precise test, to verify CARD9 mutations detected by wider panels such as NGS.

Family testing (segregation analysis)

To be sure of the pattern of inheritance, the doctors will test the parents and the siblings. Each parent would need to carry one mutation. If siblings carry the same two mutations, they would be affected or at risk as well.

Functional immune testing

When it is available, physicians will test the patient's immune cells in the lab to determine how the cells react to fungal triggers. Tests conducted this way will usually occur on a research level and include:

• Western blot or flow cytometry to analyse CARD9 protein expression in immune cells
• Assays for cytokines or ERK phosphorylation to determine the reaction of cells to the fungal components β-glucan

If these tests demonstrate a lack or poor CARD9 activity, it confirms the genetic diagnosis.

Why a confirmed diagnosis matters

Once diagnosed, patients and families can take informed steps to manage the condition.

For the patient

Treating infections

Effective antifungal treatment with systemic antifungal drugs (e.g., amphotericin B, fluconazole, echinocandins) is required. Treatment often lasts for months, sometimes simultaneously with other drugs to support the immune response, for instance.

Preventing relapse

Long-term or sometimes permanent antifungal prophylaxis is commonly advocated, as the fungal infections may recur because of the underlying persistent immune defect.

Monitoring and early intervention

Patients are advised to seek immediate care at the first sign of infection. Regular checkups help spot problems early. Some patients undergo routine MRI scans if they have had brain involvement previously.

Rare curative options

For extremely severe, recurrent instances, the immune apparatus has been replaced by the use of hematopoietic stem cell transplant (HSCT). Some individuals have been cured to date, but it is highly risky.

For the family

Genetic counseling

Awareness of the mode of inheritance allows the family to prepare. Both parents being carriers, each offspring has a 25% risk of being affected.

Carrier testing

Parents and siblings may be tested. Knowing carrier status may guide future family planning decisions.

Prenatal or preimplantation genetic diagnosis

For those couples considering further children, these would include chorionic villus sampling, amniocentesis, or IVF with genetic screening. 

Final note

An established diagnosis of CARD9 deficiency gives both doctors and patients the strength to take action. With proper diagnosis, customised treatment, and prevention measures, individuals with this condition can lead normal, productive lives — and researchers look to the future for even improved treatments.

Summary

CARD9 deficiency is an exceedingly rare genetic immunodeficiency, making individuals drastically predisposed to selected fungal infections (most notably invasive Candida, unexpected moulds, and deep fungal infections of the skin,) otherwise with a near-normal immune system. The condition results from mutations of the CARD9 gene by the autosomal recessive type, resulting in an inability of the CARD9 protein to function within fungal immune circuits. The condition is diagnosed by physicians performing genetic testing involving CARD9 gene sequencing to detect mutations within both gene copies, followed by confirming the diagnosis by confirming these mutations and, if necessary, functional studies to demonstrate the immune defect. Earliest indications, such as overwhelming fungal infections within a presumably healthy individual, should encourage genetic screening for CARD9 deficiency and related conditions. Once diagnosed, treatment consists of immediate and extended-duration anti-fungal therapy to ward off or manage infections, counselling of the family, along carrier studies. Although the condition is lethal if left untreated, an established diagnosis ushers in the correct therapy and even potential cures (e.g., stem cell transplantation for the most difficult situations).