Measles CBC Picture
The complete blood count in measles typically shows lymphopenia with decreased absolute CD4 and CD8 T cell counts, while the total white blood cell count is usually normal with increased immature band forms. 1, 2
White Blood Cell Count and Differential
- Total WBC count is typically normal in measles patients, though the differential shows characteristic changes 2
- Increased numbers of immature band forms are generally observed despite the normal total count 2
- The normal WBC count distinguishes measles from some other exanthematous illnesses where leukopenia is more prominent 2
Lymphocyte Changes
Lymphopenia is the hallmark finding and is present on hospital admission, reflecting decreased numbers of both CD4 and CD8 T cells in circulation 1
- The lymphopenia resolves quickly after the acute phase 1
- Absolute total lymphocyte counts are significantly reduced during acute measles, with mean counts of approximately 2117 cells/mm³ in severe cases compared to 4487 cells/mm³ in those who recover 3
- Lymphopenia is most marked in girls, in those with temperatures >38.5°C, and in malnourished children 1
CD4/CD8 Ratio
- CD4/CD8 ratios are decreased at all time points during and after measles infection 1
- These ratios remain lower in boys than in girls at discharge and follow-up 1
- The decreased ratio reflects proportional reductions in both CD4 and CD8 T cells 1
Platelet Count
- Thrombocytopenia may be observed in measles patients, though it is not as consistent a finding as lymphopenia 2
- Thrombocytopenia occurs at a ratio of 1 per 3,000 cases and is more likely to affect children 2
Other Laboratory Findings
- Mild elevations in hepatic transaminases may be observed 2
- Hyponatremia might be present 2
- Plasma levels of β2 microglobulin and soluble markers (Fas, Fas ligand, CD8, CD4, TNF receptor) are increased 1
Clinical Correlation and Timing
The lymphopenia and associated changes contribute to the immunosuppression characteristic of measles, which increases susceptibility to secondary infections 1, 4
- Most laboratory abnormalities have not returned to control values 1 month after discharge, consistent with prolonged immune response abnormalities 4
- The severity of lymphopenia correlates with clinical severity, with the lowest counts seen in fatal cases 3
Important Caveats
Do not confuse measles CBC findings with tickborne rickettsial diseases, where leukopenia and thrombocytopenia are more prominent features 2. The normal WBC count with lymphopenia and increased bands is more characteristic of measles, whereas conditions like HME and HGA show leukopenia in up to 53% of patients and thrombocytopenia in up to 94% 2.