Skip to content

Add your earlier progress to your account?

From before you signed in, this browser has:

Which plan do you want to keep?

You answered the plan questions before you signed in, and those answers don't match the plan saved on your account.

SettingSaved planNew answers

Study progress is waiting to be saved

Viral load and CD4 counts

14 min

Skills you'll build2 skills
  • Distinguish what an HIV viral load measures from what a CD4 count measures
  • Calculate a CD4-to-CD8 ratio from comparable absolute counts

Try first

Try first

A man has taken antiretroviral therapy for 6 months. His CD4 count rose from 280 to 410 cells/µL, and his HIV-1 RNA today is 3,400 copies/mL. Do his results show a suppressed virus?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Viral load and CD4 count

The viral load is a quantitative HIV-1 RNA test on plasma, reported in copies/mL. It measures the virus in circulation and shows how well therapy controls it.1

  • The smallest significant change between two results is threefold, or 0.5 log10 copies/mL.
  • Optimal suppression is a confirmed result below the assay's detection limit, generally below 20 copies/mL.
  • Virologic failure is HIV RNA that cannot be brought or kept below 200 copies/mL.1

The CD4 count describes immune status. A change of about 30% of the absolute count is meaningful. Marrow-suppressive drugs, acute illness and splenectomy shift the absolute count, and the CD4 percentage stays steadier in those settings.1 A rising CD4 count does not show that the virus is suppressed. Serial results for one patient are run on one platform, because values from different assays are not interchangeable.1

Absolute counts and the ratio

Flow cytometry gives each subset as a percentage of lymphocytes. A dual-platform absolute count multiplies back from the hematology count.2 Absolute subset count (cells/µL) = absolute parent-population count (cells/µL) × subset percentage ÷ 100, where the parent is the population in which the percentage was measured.

  1. Lymphocytes (cells/µL) = WBC (cells/µL) × lymphocyte percentage ÷ 100.
  2. CD4 count (cells/µL) = lymphocytes (cells/µL) × CD4 percentage ÷ 100.
  3. CD8 count (cells/µL) = lymphocytes (cells/µL) × CD8 percentage ÷ 100.

CD4-to-CD8 ratio = absolute CD4 count ÷ absolute CD8 count, with both counts in the same units from comparable measurements. The ratio is dimensionless and is not an HIV surveillance staging criterion. Both counts come from the same specimen and the same lymphocyte count, so the ratio compares like with like.2

CDC surveillance staging uses the absolute CD4 count. From age 6 years, stage 1 is 500 cells/µL or more, stage 2 is 200 to 499 cells/µL and stage 3 is fewer than 200 cells/µL. The CD4 percentage is used only when no count is available.3

Follow the bench demonstration

Sources for this step
References
  1. Panel on Antiretroviral Guidelines for Adults and Adolescents. Guidelines for the Use of Antiretroviral Agents in Adults and Adolescents with HIV: plasma HIV-1 RNA (viral load) and CD4 count monitoring. Updated September 25, 2025. Accessed September 27, 2026. https://clinicalinfo.hiv.gov/en/guidelines/hiv-clinical-guidelines-adult-and-adolescent-arv/plasma-hiv-1-rna-cd4-monitoring
  2. Mandy FF, Nicholson JKA, McDougal JS. Guidelines for performing single-platform absolute CD4+ T-cell determinations with CD45 gating for persons infected with human immunodeficiency virus. MMWR Recomm Rep. 2003;52(RR-2):1-13. Accessed September 27, 2026. https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5202a1.htm
  3. Selik RM, Mokotoff ED, Branson B, Owen SM, Whitmore S, Hall HI. Revised surveillance case definition for HIV infection: United States, 2014. MMWR Recomm Rep. 2014;63(RR-03):1-10. Accessed September 27, 2026. https://www.cdc.gov/mmwr/preview/mmwrhtml/rr6303a1.htm

Watch one

Read each decision and its explanation.

An adult with HIV has a WBC of 5,600 cells/µL with 30% lymphocytes. Flow cytometry finds 22% of lymphocytes CD3-positive/CD4-positive and 48% CD3-positive/CD8-positive.

What are the absolute CD4 and CD8 counts and the CD4-to-CD8 ratio?

  1. Lymphocytes = 5,600 cells/µL × 30 ÷ 100 = 1,680 cells/µL.

    Both subset percentages were measured within the lymphocytes, so the lymphocyte count is the parent.

  2. CD4 = 1,680 cells/µL × 22 ÷ 100 = 369.6 cells/µL.

    The CD4 percentage applies to that lymphocyte count.

  3. CD8 = 1,680 cells/µL × 48 ÷ 100 = 806.4 cells/µL.

    The CD8 percentage applies to the same parent, which keeps the two counts comparable.

  4. Ratio = 369.6 ÷ 806.4 = 0.458.

    Both counts are in cells/µL, so the units cancel and the ratio has none.

  5. Report CD4 370 cells/µL, CD8 806 cells/µL and a ratio of 0.46.

    Rounding once at the end avoids rounding error in the ratio.

  6. A CD4 count of 370 cells/µL falls in the stage 2 range of 200 to 499 cells/µL.

    Staging uses the absolute CD4 count. The ratio is not part of staging.

CD4 = 370 cells/µL, CD8 = 806 cells/µL, CD4-to-CD8 ratio = 369.6 ÷ 806.4 = 0.46.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A report gives an absolute CD4 count of 540 cells/µL and an absolute CD8 count of 1,200 cells/µL from the same specimen. What is the CD4-to-CD8 ratio, to two decimal places?

Hint
  1. The ratio puts CD4 on top.
  2. Both counts are in cells/µL, so the result has no unit.
Show the answer

0.45

Ratio = 540 cells/µL ÷ 1,200 cells/µL = 0.45. Dividing CD8 by CD4 gives 2.22, the inverted ratio.

Review CD4 count

Problem 2 of 3 · Hints available

A woman's HIV-1 RNA has stayed below 20 copies/mL. After she started a marrow-suppressive drug, her CD4 count fell from 520 to 340 cells/µL. Her CD4 percentage stayed at 28%. How are the results interpreted?

Viral control is read from the viral load, which stayed below 20 copies/mL.

A steady percentage is expected when all lymphocytes fall together. It fits the drug's effect on the marrow.

The count fell 35%, and the steady percentage shows the whole lymphocyte population fell with it. The viral load shows continued suppression.

Hint
  1. Work out how large the CD4 change is as a share of 520.
  2. Ask what the CD4 percentage is a percentage of.

Review HIV-1 RNA viral load

Problem 3 of 3 · On your own

A specimen has a WBC of 4,800 cells/µL with 25% lymphocytes. Flow cytometry finds 21% of lymphocytes CD4-positive T cells and 55% CD8-positive T cells. What is the CD4-to-CD8 ratio, to two decimal places?

Show the answer

0.38

Lymphocytes = 4,800 × 25 ÷ 100 = 1,200 cells/µL. CD4 = 1,200 × 21 ÷ 100 = 252 cells/µL. CD8 = 1,200 × 55 ÷ 100 = 660 cells/µL. Ratio = 252 ÷ 660 = 0.38. Because both subsets share one parent, 21 ÷ 55 gives the same 0.38.

Review CD4 count

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Ana Pipkin, 38, has taken antiretroviral therapy for 2 years.
  • Her monitoring results are run on the laboratory's usual platforms.
TestResultPreviousReference intervalFlag
HIV-1 RNA640 copies/mL High3 months ago: <20 copies/mL<20 copies/mL3 months ago<20 copies/mL<20High
CD4 count410 cells/µL3 months ago: 315 cells/µL315 cells/µL3 months ago
CD8 count1,025 cells/µL

Specimen: H 2, L 6, I 1. Plasma for HIV-1 RNA and whole blood, EDTA, for flow cytometry, drawn together

Decision 1 of 2

Her CD4 count rose by 30%. Which result shows how well the virus is controlled?

A meaningful CD4 rise describes her immune system. The virus itself is measured by the HIV-1 RNA.

Treated a CD4 rise as a direct measure of viral suppression

Viral load measures circulating HIV RNA. The CD4 count reflects immune status and also shifts with marrow-suppressive drugs, acute illness, and splenectomy. Reading a CD4 rise as suppression can miss virologic failure, HIV RNA that does not reach or stay below 200 copies/mL.

The ratio describes the balance of two T-cell subsets. It does not measure the virus.

The viral load measures circulating virus. It has risen from below 20 to 640 copies/mL, more than threefold.

Review HIV-1 RNA viral load

Decision 2 of 2

The ratio is 410 ÷ 1,025 = 0.40. A coworker suggests reporting that it places her in surveillance stage 3. What do you tell them?

The ratio is not used in CDC staging. Her CD4 count of 410 cells/µL is above the stage 3 limit of 200 cells/µL.

Used a subset ratio as the HIV surveillance stage

The CDC surveillance stage uses the absolute CD4 count, or the CD4 percentage when no count is available. The CD4-to-CD8 ratio plays no part in staging, and staging from it can assign stage 3 to a patient whose CD4 count is above 200 cells/µL.

CDC staging uses the absolute CD4 count. At 410 cells/µL she is in the stage 2 range.

The percentage is used only when no absolute count is available. Her count is on the report.

Review CD4 count

The result that matters is the HIV-1 RNA of 640 copies/mL. The CD4 rise and the ratio describe her immune system. Only the viral load measures the virus.

For reference

Sources checked