So CD4 counts appear to be a useful tool in assessing risk, but other factors also contribute such as lung architecture. In a retrospective study of 74 patients with interstitial lung disease on corticosteroids, 7 patients developed PCP. The mean dose at time of diagnosis was prednisone 37mg with mean duration of 10 weeks. CD4 counts ranged from 59 to 836, with a mean of 370 . The authors argued that due to their underlying lung disease, the patients were at higher risk for PCP and became infected at higher CD4 counts than patients with other underlying diseases.
Trimetrexate (Neutrexin) is a significantly more potent inhibitor of DHFR than trimethoprim (Proloprim), 18 so potent that hematopoietic cells must be protected through the coadministration of leucovorin. Although trimetrexate is significantly less toxic than trimethoprim-sulfamethoxazole, it is also less effective. 21 Because it is administered once daily, trimetrexate can be used in outpatients even though it is given intravenously. To mimic the sequential enzyme blockade provided by trimethoprim-sulfamethoxazole, dapsone (100 mg orally) can be added to the regimen.