Immunodeficiency diagnostic workup: essential preparation steps

Average diagnostic delays for primary immunodeficiency have been reported in the range of 9 to 15 years from initial symptom onset to formal confirmation. That delay reflects more than one problem: rare disease, overlapping childhood symptoms, access to specialist care, changing clinical presentations, and incomplete records all play a role.

Good preparation cannot remove every obstacle, but it can prevent avoidable repetition and give the clinical immunologist a usable diagnostic map. A structured infection history, a clear vaccination record, the original newborn-screening report, age-adjusted laboratory results, and a complete medication history often matter as much as the next advanced assay.

This framework is intended for children being evaluated for suspected IEI. The current IUIS classification includes more than 500 recognized monogenic entities, but not every child with recurrent infections has a monogenic disorder, and not every diagnosis requires identification of a pathogenic variant. The purpose of pre-consultation preparation is to help the specialist decide which possibilities deserve testing first, which results need confirmation, and which tests may be misleading in the current clinical context.

Clinical History Documentation: Building the Diagnostic Foundation

The diagnostic value of an immune test depends heavily on the clinical question behind it. A low immunoglobulin level, an unusual lymphocyte count, or a weak vaccine response cannot be interpreted in isolation. The immunologist needs to know what happened before the sample was taken, how severe the illness was, and whether the pattern is persistent.

Record infections as episodes, not impressions

“Always sick” is understandable parental shorthand, but it is difficult to use diagnostically. A dated record is more useful. For each significant infection, note:

• The child’s age and the date of onset.
• The suspected or confirmed pathogen.
• The anatomical site: ear, lung, skin, gastrointestinal tract, blood, central nervous system, or another site.
• Whether the infection was viral, bacterial, fungal, or parasitic.
• The treatment used, including route and duration.
• Whether the child required intravenous antibiotics, hospitalization, intensive care, drainage, or surgery.
• How quickly the child responded and whether the infection recurred after treatment.
• Any microbiology reports, imaging, discharge summaries, or specialist letters.

Several minor respiratory infections in a child attending daycare may be part of ordinary exposure. The pattern becomes more concerning when infections are unusually severe, persistent, difficult to treat, caused by opportunistic organisms, or located in deep tissues. Recurrent pneumonia, bacteremia, meningitis, invasive fungal disease, repeated deep abscesses, and Pneumocystis jirovecii infection carry more diagnostic weight than the number of uncomplicated colds alone.

The timing also matters. Infections beginning during the first months of life raise different questions from recurrent sinus or lung infections that appear later in childhood. A child who improves between episodes presents a different diagnostic problem from one with persistent diarrhea, chronic thrush, poor growth, or continuous viral shedding.

Map the pathogen profile

If a pathogen was cultured, detected by PCR, or identified through another validated test, keep the original report rather than relying on memory. The type of organism can help direct the first immunological questions:

• Recurrent infections with encapsulated bacteria, including Streptococcus pneumoniae or Haemophilus influenzae type b, may point toward antibody, splenic, or complement-related problems.
• Recurrent or severe mycobacterial disease and unusual Salmonella infections may raise concern for Mendelian susceptibility to mycobacterial disease and defects affecting interferon pathways.
• Persistent or severe infections with CMV, EBV, HSV, or other viruses may be associated with T-cell or NK-cell dysfunction, although the clinical context is essential.
• Invasive or unusually persistent fungal infections can suggest phagocyte dysfunction, T-cell impairment, or another form of immune dysregulation.

These associations are directional, not diagnostic. A single infection does not establish an immune defect, and the same pathogen may occur in children with very different underlying conditions.

Add growth, development, and examination clues

Bring growth records from primary care and hospital visits whenever possible. Weight loss, poor weight gain, persistent short stature, chronic diarrhea, or feeding difficulty can accompany severe or combined immunodeficiencies, but they can also result from gastrointestinal, metabolic, nutritional, or inflammatory disease.

The record should also mention:

• The onset and persistence of lymphadenopathy.
• Hepatosplenomegaly.
• Chronic oral or skin candidiasis.
• Eczema, unusual rashes, or recurrent skin infections.
• Autoimmune cytopenias or other autoimmune manifestations.
• Warts that are extensive, persistent, or difficult to treat.
• The presence or absence of tonsils and lymphoid tissue when this has been noted by a clinician.
• Developmental or neurological findings that may suggest a syndromic condition.

The absence of a classic feature is also useful. A normal growth trajectory or a normal physical examination does not exclude IEI, but it changes the weight assigned to different possibilities.

Build a three-generation family history

A family pedigree should include both affected and apparently unaffected relatives. Record:

• Consanguinity or a shared ancestral background.
• Recurrent infant deaths or unexplained childhood deaths.
• Male relatives with severe infections or early malignancy.
• Autoimmune disease, inflammatory bowel disease, or unexplained cytopenias.
• Relatives with bronchiectasis, repeated hospitalizations, or unusual infections.
• Known immune, genetic, or hematological diagnoses.
• Miscarriages or stillbirths when a clinician has suggested a possible inherited disorder.

A pattern of affected males connected through the maternal side may increase suspicion for an X-linked condition. Consanguinity may increase the likelihood of a recessive disorder, but neither observation identifies a diagnosis on its own. The pedigree helps determine which inheritance models and testing strategies are reasonable.

Document vaccination and treatment history together

List every vaccine the child has received, including approximate dates when exact dates are unavailable. Distinguish live-attenuated vaccines from inactivated, subunit, or conjugate vaccines, and record unusual reactions. Include immunoglobulin replacement therapy, blood products, and prolonged courses of immunosuppressive medication, because each can affect the interpretation of antibody testing.

The useful question is not simply how often a child is ill, but what the infections are, how severe they become, how the child recovers, and whether the same pattern appears in the family.

Safety Protocols: Managing Vaccination Status Before Testing

Live-attenuated vaccines require particular caution when a serious T-cell defect or severe combined immunodeficiency (SCID) is suspected. In these conditions, a weakened vaccine strain may cause disseminated or severe infection. The risk is not the same across all IEIs, however. A general suspicion of “immunodeficiency” is not, by itself, a universal reason to defer every live vaccine.

When deferral is relevant

If the clinical picture or newborn screening raises concern for SCID, profound T-cell lymphopenia, or another significant T-cell defect, the treating clinician should review upcoming live-attenuated vaccines before administration. Depending on the child’s age, immunological findings, local policy, and specialist advice, this may include:

• MMR.
• Rotavirus vaccine.
• Varicella vaccine.
• Live attenuated influenza vaccine given as a nasal spray.
• BCG in settings where it is part of routine vaccination.
• Oral polio vaccine where it remains in use.
• Yellow fever vaccine when travel is being considered.

The decision should be individualized. Some children with antibody deficiencies, complement defects, or milder immune abnormalities may not face the same contraindications. Conversely, a child with a confirmed or strongly suspected T-cell disorder may need urgent specialist guidance rather than routine vaccination scheduling. Families should not independently cancel or restart vaccines; they should contact the clinician responsible for immunology or primary care.

If BCG has already been given, record the date and site and report persistent inflammation, ulceration, regional lymph-node swelling, fever, or other concerning symptoms promptly. Suspected disseminated vaccine-strain disease is a clinical emergency.

Inactivated vaccines and antibody testing

Inactivated, recombinant, conjugate, and subunit vaccines are generally not subject to the same live-vaccine restrictions, although their usefulness and expected response depend on the child’s condition. Vaccine history is essential when interpreting specific antibody titers. A low titer is difficult to interpret if the child never received the relevant antigen, received it very recently, received immunoglobulin replacement, or was tested at an inappropriate interval.

When a response is being assessed, the immunologist may compare baseline and post-vaccination titers. The interval depends on the antigen and the laboratory protocol; a post-vaccination sample is often considered several weeks after immunization rather than immediately afterward. The record should state exactly which vaccine was given and when.

Household contacts

Household vaccination decisions also depend on the suspected defect. Inactivated vaccines are generally appropriate for contacts. Live vaccines may be acceptable in many situations, but some require precautions. Oral polio vaccine and smallpox-related vaccines are special cases, and the advice may differ by country and by the child’s degree of immune compromise. Families should tell the immunology team about recent live-vaccine administration in siblings or other household members rather than assuming that all exposure is harmless or all exposure is dangerous.

Interpreting Newborn Screening: TREC and KREC Insights

Newborn screening can identify infants who need rapid assessment for severe T-cell lymphopenia and, in some programs, significant B-cell abnormalities. TREC and KREC assays are screening tools. They prioritize follow-up; they do not establish a diagnosis.

What TREC results indicate

T-cell receptor excision circles, or TRECs, are formed during T-cell development in the thymus. A low or absent TREC result may indicate reduced production or an abnormally low number of newly formed T cells. This is important in SCID, but an abnormal result can also occur in other settings, including some chromosomal or syndromic conditions, congenital heart disease associated with lymphopenia, and prematurity with delayed immune maturation.

The result must be interpreted with the infant’s gestational age, clinical status, absolute lymphocyte count, and the laboratory’s cutoff values. A borderline result and an absent result do not carry the same urgency, and local screening algorithms differ.

What KREC results indicate

Kappa-deleting recombination excision circles, or KRECs, provide information about B-cell production and replication history. Some screening programs use KRECs to identify profound B-cell deficiencies, including conditions associated with X-linked agammaglobulinemia. A low KREC result is not a diagnosis and requires correlation with B-cell numbers, immunoglobulin measurements, and clinical findings.

What happens after an abnormal screen

The original report should be obtained and taken to the specialist. It should show the numerical result, the laboratory cutoff, the sample date, and any comments about the specimen. Follow-up commonly includes:

• A complete blood count with an absolute lymphocyte count.
• Flow cytometry for T, B, and NK-cell populations, often including CD3, CD4, CD8, CD19, and CD16/56.
• Repeat screening or molecular testing when recommended by the local algorithm.
• Quantitative immunoglobulins when age and clinical context make them interpretable.
• Genetic testing if the phenotype and specialist assessment support it.

Flow cytometry is usually central to confirming the type and severity of lymphopenia. Genetic analysis can help identify an underlying cause, guide treatment, and inform family counseling, but it is not universally required for diagnosis. Some patients have a clear clinical and immunological phenotype without an immediately identifiable variant; others may have variants of uncertain significance or a gene that is not included in the first test.

A positive newborn screen establishes priority, not a final label. The next step is confirmation of the immune phenotype and assessment of the child’s clinical risk.

Pre-analytical factors

The timing of collection, gestational age, transfusion history, and the quality of the dried blood spot can affect interpretation. A result from a premature infant may require a different follow-up pathway from a result in a full-term newborn. If the child has received a transfusion or undergone intensive medical treatment, tell the immunologist, because these details may affect subsequent testing.

An urgent referral should not wait for a complete family archive. When SCID is a realistic concern, infection-prevention and vaccine decisions may need to proceed while confirmatory testing is being arranged.

Baseline Laboratory Evaluation: Essential First-Line Tests

First-line investigations establish the child’s immune phenotype. They do not replace specialist assessment, but they help determine whether the next step should focus on antibody production, lymphocyte development, phagocyte function, complement, immune dysregulation, or a broader syndromic process.

Complete blood count with differential

The CBC should include absolute lymphocyte and neutrophil counts, as well as platelets and hemoglobin. Age-adjusted reference ranges are essential: a value that would be abnormal in an adult may be expected in an infant, and the reverse can also occur.

Repeated measurements are often more informative than one isolated result. Persistent lymphopenia can support concern for a T-cell or combined defect. Neutropenia may direct attention toward phagocyte disorders, marrow failure, medication effects, or cyclic patterns. Low platelets combined with eczema or recurrent infections may suggest a syndromic disorder. A normal CBC does not exclude antibody deficiency, complement disease, or some milder combined immunodeficiencies.

Quantitative immunoglobulins

IgG, IgA, IgM, and sometimes IgE should be interpreted against age-specific reference intervals. During infancy, maternal IgG declines while the child’s own antibody production develops, so adult reference ranges are inappropriate. A single low value may require repeat testing, especially if the child was acutely unwell or the result is close to the laboratory’s lower limit.

Immunoglobulin replacement therapy can make serum antibody measurements and specific antibody responses difficult to interpret. Record the product, dose when known, route, and date of the last infusion.

Specific antibody responses

When the vaccination history is reliable, antibody testing may assess responses to tetanus, diphtheria, pneumococcal serotypes, or Hib. The laboratory report should be interpreted alongside:

• The exact vaccine products received.
• The number of doses.
• The time since the last dose.
• The child’s age.
• Any immunoglobulin replacement.
• The laboratory’s definition of an adequate response.

A weak response may support a specific antibody deficiency, but it is not interpreted in isolation. Antibody concentration does not always reflect the whole of humoral immunity, and repeat assessment may be needed.

Complement studies

CH50 and AH50 can be useful when there are recurrent invasive infections with encapsulated bacteria, meningococcal disease, unusual infections, or clinical features such as angioedema. CH50 broadly assesses the classical pathway, while AH50 evaluates the alternative pathway. Abnormal results require attention to sample handling and often repeat testing, because complement proteins can be consumed during acute illness or become unreliable if the specimen is delayed or improperly transported.

Sample handling is part of the test

Ask the receiving laboratory how samples should be collected, transported, and timed. Serum immunoglobulin testing may be affected by hemolysis or lipemia. Complement studies often require rapid processing and controlled transport. Fresh blood is usually required for flow cytometry, and functional studies may have narrow stability windows.

Do not assume that a sample suitable for a routine CBC can automatically be used for every specialized immune assay. The immunology team or reference laboratory should specify the tube type, minimum volume, temperature, courier timing, and acceptable interval before analysis.

A first-line panel is not a formality. It supplies the phenotype that makes later flow cytometry, functional testing, and genetic analysis interpretable.

Coordinating Specialized Testing with Clinical Immunologists

Advanced immune testing is most useful when it answers a defined clinical question. Broad panels ordered without a phenotype can produce incidental findings, variants of uncertain significance, or results that cannot be interpreted because the sample was collected at the wrong time.

Reconcile medications and recent treatment

Provide a complete list of current and recent medicines, including:

• Systemic corticosteroids and other immunosuppressants.
• Calcineurin inhibitors, mycophenolate, or azathioprine.
• Biologic agents such as rituximab or cytokine inhibitors.
• Chemotherapy.
• Immunoglobulin replacement.
• Blood products and transfusions.
• Antimicrobial prophylaxis when relevant.

These treatments can change lymphocyte counts, B-cell numbers, antibody concentrations, or T-cell proliferation. Testing during treatment is not automatically useless, but the result may describe the treatment effect rather than the child’s untreated immune system. The specialist may recommend a different timing, add a complementary assay, or interpret the result with caution.

Describe illness at the time of sampling

Acute infection can transiently change lymphocyte counts, inflammatory markers, and functional immune responses. Sepsis or severe viral infection may make some functional assays difficult to interpret, but it does not categorically invalidate every assay. In urgent situations, testing during illness may be necessary and clinically valuable.

When the condition is stable, the team may prefer to repeat selected studies after recovery. The appropriate interval depends on the infection, the suspected disorder, and the urgency of the decision. Rather than applying a fixed waiting period to every child, document the date of symptom resolution and ask the laboratory or immunologist which tests can proceed.

Plan flow cytometry and functional assays carefully

Flow cytometry may examine T, B, and NK-cell numbers, maturation markers, memory and naïve populations, activation markers, or protein expression relevant to a suspected disorder. Functional assays may evaluate lymphocyte proliferation, oxidative burst, cytotoxicity, cytokine production, or other pathway-specific responses.

These studies differ in sample requirements. Some need same-day processing; others can be transported to a reference center under defined conditions. Before the blood draw, confirm:

• The receiving laboratory.
• The permitted transport temperature.
• The latest arrival time.
• Whether the test is performed on weekdays only.
• Whether the child should avoid a particular medication.
• Whether a repeat sample may be needed.

Prepare for genetic testing without treating it as the entire diagnosis

Genetic testing may include a targeted IEI panel, a broader exome-based approach, genome sequencing, or testing for a specific suspected condition. The choice depends on the clinical phenotype, prior results, family structure, availability, cost, and the laboratory’s capabilities.

A targeted panel can be efficient when the immune phenotype strongly suggests a limited group of genes. Exome sequencing may be more appropriate for atypical, syndromic, or unexplained presentations, but it can identify more variants of uncertain significance and may not resolve every case. A negative genetic result does not necessarily exclude IEI.

Before testing, assemble:

• The three-generation pedigree.
• Results from previous immune and genetic investigations.
• Reports from relevant relatives.
• Information about consanguinity.
• The feasibility of testing both biological parents.
• Consent preferences regarding secondary findings, where applicable.
• Any stored DNA or tissue from an affected family member.

Trio testing can help clarify de novo and recessive variants, but it is not possible or necessary in every family. Genetic findings must be interpreted alongside the clinical phenotype and immune function; a variant alone is not proof that it explains the child’s illness.

Consider oncology and transplant history

Children receiving chemotherapy, hematopoietic stem-cell transplantation, or treatment for immune dysregulation require a specialized plan. Conditioning regimens, graft-versus-host disease, immunosuppressive drugs, infections, and reconstitution after transplant can all alter immune measurements. The laboratory should know the treatment timeline, current drug levels when relevant, donor and graft information, and the timing of any transplant-related complication.

What to bring to the consultation

A practical folder—paper or digital—should contain:

1. A dated infection timeline, including hospitalizations and microbiology.

2. Growth charts and relevant imaging or discharge summaries.

3. The complete vaccination record, with attention to live-attenuated vaccines.

4. The original TREC or KREC newborn-screening report, if available.

5. CBCs, immunoglobulin results, antibody titers, complement studies, and previous flow cytometry.

6. A medication and treatment list, including immunoglobulin replacement and transfusions.

7. A family pedigree and records of relatives with relevant disease.

8. Prior genetic reports, including findings labeled as uncertain.

9. Details of the child’s condition on the day samples were collected.

If the child is currently septic, struggling to breathe, dehydrated, severely lethargic, or showing signs of an invasive infection, emergency treatment takes priority over assembling records. The diagnostic workup can be organized in parallel.

Preparation is not a substitute for clinical judgment, and it cannot guarantee a rapid molecular answer. Its value is more concrete: it reduces missing context, prevents inappropriate assumptions about vaccine safety, helps distinguish screening from confirmation, and allows each test to be chosen for a reason. In pediatric immunology, that discipline is often what turns a long sequence of disconnected results into a coherent diagnostic investigation.