How do I order a CD4 (Cluster of Differentiation 4) count?

How to Order a CD4 Count

Order a CD4 count by requesting "CD4+ T-cell absolute count and percentage" from your laboratory, specifying that you need both CD3+CD4+ T-cells enumerated by flow cytometry with CD45 gating. 1

Specific Test Ordering Details

What to Request on the Laboratory Requisition

• Request both absolute CD4+ T-cell count AND CD4 percentage - both values must be reported together as they provide complementary clinical information 1
• Specify "CD3+CD4+ T-cells" to ensure the laboratory reports only T-helper cells (cells positive for both CD3 and CD4), not other CD4-expressing cells like monocytes 1
• Request the test be performed using flow cytometry with CD45/side-scatter gating for lymphocyte identification, which is the CDC-recommended standard method 1

Blood Collection Requirements

• Collect blood in EDTA (purple-top) tubes for flow cytometry analysis 1
• The specimen should be processed within 30 hours, but no later than 48 hours after collection for optimal results 1
• Ensure the laboratory receives the specimen with adequate time for processing within this window 1

Understanding What the Laboratory Will Report

Standard Reporting Format

• The laboratory will report CD4+ T-cell absolute count (cells/μL or cells/mm³) calculated using the formula: (% CD3+CD4+ cells) × (% lymphocytes) × (white blood cell count) ÷ 100 1
• The laboratory will also report CD4 percentage (%), which represents the proportion of CD3+CD4+ cells among total lymphocytes 1
• Reference ranges will be laboratory-specific and should be established separately for adults and children 1

Clinical Interpretation Thresholds

• CD4 percentage <14% typically corresponds to absolute CD4 count <200 cells/μL, indicating significant immunosuppression 2, 3
• CD4 percentage ≥27% typically corresponds to absolute CD4 count ≥500 cells/μL 3
• These percentage correlations are useful when complete blood count data is unavailable 3

Common Pitfalls to Avoid

Timing Issues

• Do not order CD4 counts during acute illness - CD4 counts and percentages vary substantially during acute illness; obtain measurements when the patient is clinically stable 2
• Avoid ordering the test if the specimen cannot reach the laboratory within the 48-hour processing window 1

Interpretation Errors

• Do not confuse CD4+ T-cells with total CD4-expressing cells - the clinically relevant value is CD3+CD4+ (T-helper cells only), not all CD4-expressing cells which include monocytes and other cell types 1
• Do not rely on CD8 counts for routine HIV monitoring - CD8 cell count measurement is not recommended for routine clinical decision-making in HIV management 2
• Be aware that absolute lymphocyte count (ALC) <1,000 cells/mm³ predicts CD4 count <200 cells/mm³ with 91% positive predictive value, but this is not a substitute for actual CD4 testing 4

Quality Considerations

• Ensure your laboratory follows CDC guidelines requiring ≥85% lymphocyte purity (optimally ≥90%) within the lymphocyte gate for accurate results 5
• The laboratory should use CD45/side-scatter gating to achieve >95% lymphocyte purity without requiring correction factors 1