Before We Start
The exact number changes the differential
Rather than treating 'immunocompromised' as one vague risk category, each CD4 threshold unlocks a specific, predictable set of new infection risks — which is exactly why exam vignettes always give you a specific number, not just 'low CD4.'
💡 Memory Trick
HIV/AIDS CD4 thresholds: below 500 (early), below 200 (PCP, AIDS-defining), below 100 (Toxo, Crypto), below 50 (MAC, CMV). Each threshold crossed unlocks a genuinely new set of risks.
The Key Points
Four thresholds, and the specific infections that appear at each
<500
CD4 below 500 — early symptomatic disease
As CD4 counts fall below 500, patients begin to develop oral candidiasis, hairy leukoplakia (caused by EBV), and recurrent bacterial infections — relatively early signs of immune decline, well before the more severe AIDS-defining thresholds are reached.
🦠 A patient with a CD4 count of 450 developing oral candidiasis and hairy leukoplakia reflects this early stage of immune decline, distinct from the more severe opportunistic infections that appear at lower thresholds.
<200
CD4 below 200 — the AIDS-defining threshold
Below 200, PCP (Pneumocystis pneumonia) becomes a real risk, and this threshold triggers starting TMP-SMX prophylaxis — which conveniently also provides Toxoplasma prophylaxis at the same time, covering two future risks with one medication.
🦠 A patient whose CD4 count drops to 180 should be started on TMP-SMX prophylaxis specifically because it addresses both the immediate PCP risk and provides overlapping protection against future Toxoplasma reactivation.
<100
CD4 below 100 — Toxoplasmosis and Cryptococcal meningitis
Below 100, Toxoplasmosis and Cryptococcal meningitis become significant risks — both organisms capable of establishing dormant infection in a healthier immune system, only reactivating once immune surveillance drops this low.
🦠 A patient with a CD4 count of 85 who develops a headache should specifically raise concern for either Toxoplasma reactivation or cryptococcal meningitis, given this exact threshold.
<50
CD4 below 50 — the most severe immunosuppression
Below 50, MAC (Mycobacterium avium complex) becomes a risk, triggering azithromycin prophylaxis, and CMV retinitis (presenting with floaters and decreased vision) becomes a concern as well — the lowest threshold, unlocking the most severe range of opportunistic risks.
🦠 A patient with AIDS and a CD4 count of 45 develops new floaters and decreased vision — this points to CMV retinitis, an opportunistic infection specifically associated with this severe level of immunosuppression, requiring urgent ophthalmologic evaluation.
🏥 Applied Scenario
A patient with AIDS has a CD4 count that has fallen to 45 and reports new floaters along with decreased vision in one eye.
Step 1
Identify what this combination points to: What does this combination of CD4 level and symptoms point to? CMV retinitis — an opportunistic infection specifically associated with CD4 counts below 50, and floaters plus decreased vision are its classic presenting symptoms.
Step 2
Contrast with a different CD4 level and symptom set: If this same patient's CD4 count were instead around 150 rather than 45, and they developed progressive dyspnea rather than visual symptoms, PCP would be the much more likely concern instead — reflecting the different threshold (below 200) associated with that infection.
Step 3
Recognize the reasoning approach exams expect: This threshold-based approach — using the specific CD4 number to narrow down which opportunistic infection is most likely — is exactly how exams expect you to reason through an HIV-related infection vignette, rather than just recognizing "immunocompromised" as a vague risk category.
Step 4
Conclusion: The exact CD4 number given in a vignette is never incidental — it's the specific detail meant to narrow the differential to one particular opportunistic infection.
📌 Exam Application
Exams test matching each CD4 threshold to its associated opportunistic infections (below 500: candidiasis/hairy leukoplakia; below 200: PCP, AIDS-defining, TMP-SMX started; below 100: Toxoplasma/Cryptococcus; below 50: MAC/CMV retinitis) and the specific prophylaxis started at each relevant threshold.
⚠️ The Trap — Forgetting TMP-SMX's Dual Prophylactic Coverage
The most common trap is forgetting that TMP-SMX prophylaxis, started at the CD4 <200 threshold for PCP, also happens to cover Toxoplasma prophylaxis at the same time — meaning a patient already on TMP-SMX for PCP prevention is getting overlapping protection against a risk that technically wouldn't appear until CD4 drops even further (below 100).
✓ Quick Self-Test
Answer before checking:
1. What opportunistic conditions appear as CD4 falls below 500?
2. What infection risk begins at CD4 below 200, and what prophylaxis is started?
3. What two infections become significant risks at CD4 below 100?
4. What two conditions become risks at CD4 below 50, and what prophylaxis is started for one of them?
5. What are the classic presenting symptoms of CMV retinitis?
Answers:
1. Oral candidiasis, hairy leukoplakia (EBV), and recurrent bacterial infections.
2. PCP (Pneumocystis pneumonia); TMP-SMX prophylaxis is started, which also covers Toxoplasma prophylaxis.
3. Toxoplasmosis and Cryptococcal meningitis.
4. MAC (Mycobacterium avium complex, triggering azithromycin prophylaxis) and CMV retinitis.
5. Floaters and decreased vision.