Disease due to disseminated Mycobacterium avium complex (MAC) challenges the diagnostic skills of even HIV-experienced clinicians. Patients with advanced HIV disease and CD4 counts below 75 cells/mm³ are particularly susceptible to MAC, especially since the wide institution of prophylaxis for PCP. Prophylaxis for PCP reduces the likelihood that AIDS patients will contract or succumb to pneumocystosis, but in so doing increases the likelihood that they will develop other opportunistic infections, among them MAC.

Unfortunately, patients with advanced HIV disease and CD4 counts below 75 cells/mm³ are susceptible to a number of OIs in addition to MAC -- and many of these infections can present with symptoms similar to MAC: weight loss, persistent fevers, night sweats, diarrhea, fatigue, and generalized wasting. Because these symptoms are nonspecific, they can easily be confused with the equally insidious, equally debilitating presentations of other late-stage OIs.

In cases of suspected MAC disease the clinician's diagnostic conundrum is complicated by the fact that many late-stage AIDS patients suffer from multiple concurrent infections. As a result, the discovery that such a patient is culture- or biopsy-positive for cytomegalovirus, for example, does not mean that this is the sole source of that patient's symptoms. Co-infection with MAC remains a distinct possibility, and for this reason the clinician would be wise to include MAC in the differential diagnosis every time an AIDS patient presents with symptoms such as weight loss, night sweats, persistent fever, diarrhea, and fatigue.

Unfortunately, it can take two to four weeks to confirm a diagnosis of MAC by the current state-of-the- art blood culture system, and this laboratory test may not be available to all clinical labs. Desperately sick and debilitated patients cannot wait weeks, untreated, for confirmation that MAC is the source of their symptoms, and many clinicians therefore choose to treat suspected cases of MAC empirically, much as they treat suspected PCP. Given the high levels of morbidity and mortality associated with disseminated MAC disease, prompt treatment of probable infection is both rational and compassionate. The following algorithm will assist clinicians in reaching a preliminary diagnosis, ordering laboratory tests to exclude or confirm the presence of certain pathogens, and initiating effective treatment.

Diagnostic Algorithm: When to Suspect MAC Patients at High Risk Those with CD4 counts below 75 cells/mm3 and a previous history of other OIs such as CMV and PCP Typical Presenting Symptoms Constellation of: weight loss (greater than 10-15 pounds), persistent fever, diarrhea, night sweats, anemia, fatigue, generalized wasting Differential Diagnosis CMV disease (retinitis, colitis, or hepatitis); disseminated histoplasmosis, cryptococcosis, or coccidioidomycosis; TB (many cases are extrapulmonary); lymphoma; disseminated Pneumocystis carinii infection What Laboratory Tests Should Be Done Dilated eye exam, cryptococcal antigen, PPD and controls, chest radiograph, coccidioidomycosis serologies, histoplasma antigen (urine and serum), stool examinations (colonoscopy if stool examinations are negative), lymph node aspiration or biopsy (if lymphadenopathy is present), liver function tests, bone marrow biopsy (if cytopenia is present) When to Suspect MAC When symptoms described above persist for more than 7-10 days and no other diagnosis has been made When to Treat When symptoms persist and laboratory tests rule out other OIs How to Treat Clarithromycin (500 mg b.i.d.) plus ethambutol (800-1600 mg/day based on body weight)