Q: 54 year old male with HIV admitted to ICU with Respiratory failure. Initial work up showed LDH level of 1200 IU/L. What is your probable diagnosis?

Answer: Histoplasmosis

An LDH level of more than 450 IU/L is 9.33 times more likely to be associated with a diagnosis of histoplasmosis than with PCP. An LDH level of 450 IU/L or greater had a sensitivity and specificity of 70% and 80%, respectively; a value of 600 IU/L or greater had sensitivity and specificity of 50% and 89%.

Thus, serum LDH levels of 600 IU/L or greater are suggestive of histoplasmosis rather than PCP in appropriate clinical settings. 

Reference:

Butt AA, Michaels S, Greer D, Clark R, Kissinger P, Martin DH (July 2002). "Serum LDH level as a clue to the diagnosis of histoplasmosis". AIDS Read 12 (7): 317–21.