Step by Step
<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.
<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.
<100
CD4 below 100
Below 100, Toxoplasmosis and Cryptococcal meningitis become significant risks.
<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.
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 Walkthrough
1
A patient with AIDS has a CD4 count that has fallen to 45 and reports new floaters along with decreased vision in one eye.
2
Ask: 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.
3
Contrast: 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 (<200) at which that particular infection becomes relevant.
4
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.
Exam Application
Exams test matching each CD4 threshold to its associated opportunistic infections (<500: candidiasis, hairy leukoplakia; <200: PCP, prophylaxis started; <100: Toxoplasmosis, Cryptococcal meningitis; <50: MAC, CMV retinitis) and specifically that TMP-SMX prophylaxis started at CD4 <200 covers both PCP and Toxoplasma simultaneously.
⚠ Common Trap
The most common trap is forgetting that TMP-SMX prophylaxis, started at the CD4 <200 threshold for PCP, also covers Toxoplasma prophylaxis at the same time — a two-for-one detail that's frequently tested and easy to overlook if you think of PCP and Toxoplasma prophylaxis as two separate decisions.
✓ Quick Self-Check
1. What opportunistic conditions appear as CD4 falls below 500?
Oral candidiasis, hairy leukoplakia (EBV), and recurrent bacterial infections.
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2. What infection risk begins at CD4 below 200, and what prophylaxis is started?
PCP (Pneumocystis pneumonia); TMP-SMX prophylaxis is started, which also covers Toxoplasma prophylaxis.
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3. What two infections become significant risks at CD4 below 100?
Toxoplasmosis and Cryptococcal meningitis.
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4. What two conditions become risks at CD4 below 50, and what prophylaxis is started for one of them?
MAC (Mycobacterium avium complex, triggering azithromycin prophylaxis) and CMV retinitis.
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5. What are the classic presenting symptoms of CMV retinitis?
Floaters and decreased vision.
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