What this test measures
This flow-cytometry panel enumerates the main T-lymphocyte subsets by their surface markers: CD3 (all mature T cells), CD4 (helper T cells, "the orchestrators" of immunity), and CD8 (cytotoxic T cells that kill infected cells). The lab reports absolute counts (cells/µL) and percentages of total lymphocytes for each subset, plus the CD4:CD8 ratio.
The normal adult ratio of CD4 to CD8 is roughly 1.5–2.5. HIV destroys CD4 cells preferentially, inverting the ratio (CD8 > CD4) and progressively lowering absolute CD4. Outside HIV, CD4/CD8 changes also occur in primary immunodeficiency, post-transplant immunosuppression, certain infections, and lymphoid malignancies.
Why it matters
CD4 count is the central laboratory marker in HIV care. In India, the National AIDS Control Organization (NACO) and WHO guidelines use CD4 to: (1) start opportunistic-infection prophylaxis (cotrimoxazole at CD4 < 350; antifungals at CD4 < 100); (2) assess immune recovery on antiretroviral therapy (ART); and (3) define AIDS (CD4 < 200/µL or AIDS-defining illness). Although ART is now started in all HIV-positive patients regardless of CD4, the count remains essential for prognosis, prophylaxis, and identifying immune-recovery failure.
Outside HIV, T-cell subsets are used in: suspected primary immunodeficiency (low CD4 in newborns prompts SCID workup); post-stem-cell-transplant immune reconstitution monitoring; and evaluation of severe or unusual infections. The CD4:CD8 ratio also rises with age, in autoimmune disease, and falls in chronic viral infection and after BCG vaccination.
How to prepare
No fasting required. Sample stability is critical — most labs require the test be done within 24 hours of collection at room temperature. Inform the lab if you have recently had an acute infection (transient lymphopenia) or are on steroids, chemotherapy, or other immunosuppressants. Continue all HIV antiretroviral medications.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| CD3 Absolute Count (cells/µL) | 700 – 2100 | Total T-cell deficiency; severe immunosuppression, advanced HIV, drug-induced, or rare primary immunodeficiency. | Reactive lymphocytosis, viral infection, T-cell leukaemia or lymphoma. |
| CD4 Absolute Count (cells/µL)[1][2][3] | 500 – 1500 | < 500: mild immunosuppression. < 350: NACO/WHO threshold for cotrimoxazole prophylaxis. < 200: AIDS-defining; high risk of PCP, toxoplasma, cryptococcus. < 50: very high risk of CMV and atypical mycobacteria. Also seen in chemotherapy, stem-cell transplant, severe sepsis, idiopathic CD4 lymphocytopenia. | Mild elevation seen in reactive states. Marked elevation rare — consider T-cell lymphoma or specific viral infections. |
| CD8 Absolute Count (cells/µL) | 200 – 1100 | Generalised immunosuppression — chemotherapy, advanced disease, primary immunodeficiency. | Viral infection (EBV, CMV, HIV), HTLV infection, T-cell lymphoma, autoimmune disease. |
| CD4:CD8 Ratio (Ratio) | 1.5 – 2.5 | Inverted (< 1) — HIV (classic finding), advanced age, chronic CMV infection, autoimmune disease, post-transplant immunosuppression. Persistent inversion on ART suggests incomplete immune recovery. | Elevated (> 3) — autoimmune disease (Sjögren, SLE), sarcoidosis, certain T-cell malignancies. |
CD4 count thresholds in HIV care
| CD4 count (cells/µL) | Stage | Risks and recommendations |
|---|---|---|
| > 500 | Preserved | Low opportunistic infection risk |
| 350 – 500 | Mild immunosuppression | Start cotrimoxazole prophylaxis (NACO) |
| 200 – 350 | Moderate | PCP prophylaxis essential; vigilance for early opportunistic infections |
| 100 – 200 | AIDS-defining (severe) | High PCP, Toxoplasma, Cryptococcus risk |
| < 50 | Critical | CMV, MAC, severe wasting — multiple prophylaxes |
Frequently asked questions
How often should CD4 count be checked in HIV?
NACO guidance: at diagnosis, then every 6 months in early stages or until two consecutive counts > 500 on stable ART. Once stable on ART with good viral suppression, routine CD4 monitoring is less frequent — viral load becomes the main test.
What CD4 level defines AIDS?
A CD4 count < 200 cells/µL is AIDS-defining, even without an opportunistic infection. The diagnosis is also made by certain AIDS-defining illnesses (PCP, oesophageal candidiasis, Kaposi sarcoma) at any CD4 count.
How quickly does CD4 recover on ART?
Most patients see a 100–150 cell/µL rise in the first 6 months of effective ART, then a slower rise. About 10–15% have poor immune recovery despite good viral suppression — they need continued opportunistic-infection prophylaxis.
Why is the CD4:CD8 ratio important?
The ratio reflects the balance of helper and cytotoxic T cells. HIV inverts it (CD8 > CD4). Persistent inversion despite ART is associated with higher mortality, chronic inflammation, and non-AIDS complications (cardiovascular disease, certain cancers).
Are CD4 counts affected by other infections?
Yes — transient lymphopenia is common during any acute infection (including TB, dengue, sepsis), and chronic infections like TB, hepatitis C, and malaria can lower CD4. Repeat after recovery for a true baseline.
Outside HIV, when is this test used?
Primary immunodeficiency workup (especially in babies with recurrent infection — SCID screening); monitoring after bone-marrow or solid-organ transplant; severe lymphopenia of unclear cause; immune monitoring during chemotherapy; certain autoimmune diseases.
Does the sample need to be fresh?
Yes — T cells lose viability quickly. Most labs require flow cytometry within 24 hours of collection. The sample is collected in EDTA and kept at room temperature, never frozen.
Related Immunology tests
Tests commonly ordered alongside CD3CD4CD8, or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — CD4 Lymphocyte Count · accessed 2026-05-30T00:00:00.000Z
- WHO — Consolidated HIV Treatment Guidelines · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — CD4 Count · accessed 2026-05-30T00:00:00.000Z
- NACO — National Guidelines on ART (India) · accessed 2026-05-30T00:00:00.000Z
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