When I say bat poop, caves, and Indiana, the answer is Histoplasmosis. In general, most people will not get disease even when inhaling any conidia, however those who have decreased cellular immunity (i.e. solid organ transplant and HIV) are at risk of pulmonary or disseminated disease. One of the difficult issues in those patients is diagnosis. Think about the options here:
- Culture – takes way to long
- Antibody testing – is a reasonable option, but given the lack of cellular immunity in these patients, you are risking false negatives
- Histopathology – yeah, good luck trying to poke someone who is super sick (typically, but a lot of these procedures are invasive. That is, unless you have some skin findings, but this is typically seen in blastomyces).
As such, the antigen test has been used for the immunosuppressed patients. This was first described by Joseph Wheat (word of caution, a lot of the papers cited here are from this one guy) back in the mid-80s and since then, it has taken the world of ID by storm. One of the earlier studies found that urine histoplasma antigen was positive in 20/22 episodes of disseminated histoplasmosis, compared to 6/32 episodes of self-limited infections (1). Furthermore, 11/22 serum samples were positive for antigen in similar cases. A review of several diagnostic modalities found that histoplasma antigen tended to have favorable sensitivities for all types of histoplasmosis, except for subacute pulmonary histoplasmosis (2):
Notably, the vast majority of the studies tend to be on HIV patients, as the sensitivity of antigen tends to be higher in these patients as well as those with other types of immunocompromise. A multicenter retrospective study (3) found the sensitivity in immunocompromised patients to be higher than in non-immunocompromised:
Moreover, patients with AIDS and those who were immunocompromised had higher mean antigen levels (13.8 ng/mL for AIDS vs 10.48 ng/mL for immunocompromised patients vs 6.92 ng/mL). Several types of antigen assays are available, though the 2 major classifications tend to be polyclonal antibody assays vs monoclonal antibody assays (essentially, these antibodies “sandwich” the antigen, leading to detection). These polyclonal assays tend to have higher sensitivity than monoclonal assays, with polyclonal assays having a sensitivity of 90.5% and specificity of 96%, compared to monoclonal assays with 62% and 79%, respectively (4). The most common test is the MiraVista lab antigen test, however this is a sendout and its not commercially available. The ImmunoMycologics lab test, a commercially available test, has been evaluated in comparison to the MiraVista test. In a prospective study of 1003 urine specimens, there was a 97.6% agreement between the assays (5). The sensitivity of the Immuno Mycologics assay was 64.5% and specificity was 99.8% when compared to the MiraVista assay, suggesting both are fairly equivalent:
Monitoring Histoplasma Antigen
This is where the test truly shines. It is difficult enough to diagnose, and therapy tends to be quite long (including a 2 week induction period followed by a 12 month period of PO therapy for disseminated disease) and antigen testing allows a simple way of monitoring patients. Both urine and serum antigens tend to be equivalent in terms of monitoring, with several studies actually showing higher urine antigen levels when compared to serum antigen levels. For instance, one study of 601 paired serum and urine antigen samples found the concordant rate was around 98% with only 11 having discrepant results (6). A single urinary histo antigen yielded a higher positivity rate (17%) compared to paired serum and urinary antigen test (9.4%), suggesting that urinary antigen is a superior test when compared to serum antigen.
The utility of antigen for monitoring was demonstrated in an animal study of 15 cats with histoplasmosis (7). Urine histoplasma antigen was positive in 14/15 cats with a median concentration of 8.71ng/mL (0.4-19. ng/mL), with 9/11 cats having negative urine histo antigen at the time of remission compared to 6/8 for serum antigen:
The elimination of urine antigen occured at a median of 13.6 weeks (3.9-31.6) while serum antigen elimination occurred at a median of 12.7 weeks (4.7-28.4). Notably, however, the time to urine antigen elimination was shorter than time to clinical remission (23.9 weeks). Using urine antigen for remission had a sensitivity of 90% (95% CI 72.3-97.4) and specificity of 64.6% (95% CI 51.7-75.8). This has also been seen in human studies, with one study showing a downwards trend in urine histo antigen (4):
In an open-label non-randomized trial (8) evaluating itraconazole for prevention of histoplasmosis relapse found that antigen levels in both serum and urine decreased during the study period, with 35/39 patients having urine antigen detected at the beginning of the study compared to 22/39 at week 52 (p=0.004). This was a decrease from 6.91U (SD 5.62) to 3.52U (SD 4.53 p=0.0002).
Given that both serum and urine histoplasma antigen are equivalent (for the most part), and urine antigen tends to have higher levels, then the latter can be used for tracking disease. The biggest drop seems to occur within the first 2 weeks, with the rate of drop being more consistent thereafter. Data from 2 multicenter trials (amphotericin B treatment, MSG-29; itraconazole treatment, ACTG-120) evaluated antigen levels during therapy for disseminated histoplasmosis (9). Mean baseline antigen levels were 26.2ng/ML in urine samples compared to 20.6 ng/L in the serum. As noted below, antigen clearance occurred faster in the first 2 weeks of therapy (mean reduction 11.26 ng/mL in serum vs 7.65 ng/mL in urine), though afterwards, serum antigen levels cleared faster than urine antigen levels ( -5.90 ng/mL per week vs -4.21 ng/mL per week):
By the end of therapy, a significant reduction occurred in urine samples of 85.7% of patients compared to 77.8% in serum samples. Furthermore, antigen clearance tended to occur faster when treatment was instituted with amphotericin B when compared to itraconazole.
Another analysis of 2 studies demonstrated similar drops in both urine and serum antigen levels within a month, as well as in 12 weeks (10). Here therapy with fluconazole and itraconazole for disseminated histoplasmosis found that comparing paired samples at baseline with either week 4 or week 12, there was a significant decrease in both serum and urine antigen levels:
The mean drop after 4 weeks in serum was 3.46U (SD 3.84) in the itraconazole group vs 2.57 (SD 3.65U) in the fluconazole group, compared to 2.57 (SD 3.65) in the urine itraconazole group and 1.30 (SD 2.31U) in the fluconazole group. Higher drops were seen at week 12 in both groups. This suggests that the expected drop in the treatment of histoplasmosis is around 2-4U around this time. In an early study (11) comparing fungal clearance with treatment with amphotericin B and itraconazole, antigen levels in serum dropped a median of 1.6U in the liposomal amphotericin B group compared to just 0.1U in the itraconazole group (p=0.02).
Urine antigen levels dropped by 2.1U in the amphotericin group compared to 0.2U in the itraconazole group (p=0.0005) by week 2.
By week 12, 17/29 patients and 8/15 patients in the amphotericin and itraconazole group, respectively, had levels below the limit of detection. In comparison, 6/28 vs 6/32 urine antigen assays were below the limit of detection by week 12. Its utility seems to be greatest for AIDS patients. For instance, in one study (12) the detection rates in AIDS patients with disseminated histoplasmosis was 96.7% for urine antigen vs 78.7% in serum antigen. Treatment with amphotericin B reduced the urine antigen levels in 90.5% of urine samples compared to 100% of tested serum samples.
A rise in 2 units following a drop suggests relapse. A retrospective study (13) evaluated urinary antigen levels in 17 patients with relapsed histoplasmosis and 34 controls and found that in patients with relapsed histoplasmosis, antigen level increased by at least 2 units in 17 out of 18 cases in the urine and by 2 units in 12 of 14 cases in serum:
Sensitivity of urinary antigen for diagnosis of relapse was 93.8% (95% CI 69 to 99.9%) while for serum antigen was 83% (95% CI 51 to 97%). Furthermore, using the control group, the specificity of a 2 unit increase for urinary antigen levels was calculated to be 97.3% (95% CI 83 to 97%). How often and how long should you monitor? The IDSA/HIVMA (14) does mention monitoring but does not mention the frequency. One reasonable approach is to check urine histo antigen monthly for the first 3 months. This is based on an analysis of 2 multicenter trials (9), with the vast majority of patients responding within the first 3 months in terms of a drop in urine antigen in patients treated with either amphotericin B or itraconazole:
Beyond this, checking monthly to every 3 months is also a reasonable approach. After the first year, UpToDate recommends checking every 6 months for an additional one to 2 years.
- Histoplasma antigen testing has a high sensitivity for diagnosis of histoplasmosis, especially in immunosuppressed patients and in disseminated histoplasmosis
- Urine histo antigen is as good, or perhaps better than serum histoplasma antigen
- The biggest drop in urine histo occurs within the first 2-4 weeks of treatment
- Monitoring it monthly at the beginning of treatment is reasonable for the first 3-6 months, then every 3 months thereafter
- You can track urine histo antigen for relapses
- Wheat LJ, Kohler RB, Tewari RP. Diagnosis of disseminated histoplasmosis by detection of Histoplasma capsulatum antigen in serum and urine specimens. N Engl J Med. 1986 Jan 9;314(2):83-8. doi: 10.1056/NEJM198601093140205. PMID: 3941695.
- Azar MM, Hage CA. Laboratory Diagnostics for Histoplasmosis. J Clin Microbiol. 2017 Jun;55(6):1612-1620. doi: 10.1128/JCM.02430-16. Epub 2017 Mar 8. PMID: 28275076; PMCID: PMC5442517.
- Chadi A. Hage, Julie A. Ribes, Nancy L. Wengenack, Larry M. Baddour, Maha Assi, David S. McKinsey, Kassem Hammoud, Daisy Alapat, N. Esther Babady, Michelle Parker, DeAnna Fuller, Aliya Noor, Thomas E. Davis, Mark Rodgers, Patricia A. Connolly, Boutros El Haddad, L. Joseph Wheat, A Multicenter Evaluation of Tests for Diagnosis of Histoplasmosis, Clinical Infectious Diseases, Volume 53, Issue 5, 1 September 2011, Pages 448–454, https://doi.org/10.1093/cid/cir435
- Zhang X, Gibson B Jr, Daly TM. Evaluation of commercially available reagents for diagnosis of histoplasmosis infection in immunocompromised patients. J Clin Microbiol. 2013 Dec;51(12):4095-101. doi: 10.1128/JCM.02298-13. Epub 2013 Oct 2. PMID: 24088857; PMCID: PMC3838024.
- Theel ES, Jespersen DJ, Harring J, Mandrekar J, Binnicker MJ. Evaluation of an enzyme immunoassay for detection of Histoplasma capsulatum antigen from urine specimens. J Clin Microbiol. 2013 Nov;51(11):3555-9. doi: 10.1128/JCM.01868-13. Epub 2013 Aug 21. PMID: 23966508; PMCID: PMC3889733.
- Libert D, Procop GW, Ansari MQ. Histoplasma Urinary Antigen Testing Obviates the Need for Coincident Serum Antigen Testing. Am J Clin Pathol. 2018 Mar 7;149(4):362-368. doi: 10.1093/ajcp/aqx169. PMID: 29462251.
- Hanzlicek AS, Meinkoth JH, Renschler JS, Goad C, Wheat LJ. Antigen Concentrations as an Indicator of Clinical Remission and Disease Relapse in Cats with Histoplasmosis. J Vet Intern Med. 2016 Jul;30(4):1065-73. doi: 10.1111/jvim.13962. Epub 2016 May 9. PMID: 27158815; PMCID: PMC5084835.
- Wheat J, Hafner R, Wulfsohn M, Spencer P, Squires K, Powderly W, Wong B, Rinaldi M, Saag M, Hamill R, Murphy R, Connolly-Stringfield P, Briggs N, Owens S; National Institute of Allergy and Infectious Diseases Clinical Trials and Mycoses Study Group Collaborators. Prevention of relapse of histoplasmosis with itraconazole in patients with the acquired immunodeficiency syndrome. Ann Intern Med. 1993 Apr 15;118(8):610-6. doi: 10.7326/0003-4819-118-8-199304150-00006. PMID: 8383934.
- Hage CA, Kirsch EJ, Stump TE, Kauffman CA, Goldman M, Connolly P, Johnson PC, Wheat LJ, Baddley JW. Histoplasma antigen clearance during treatment of histoplasmosis in patients with AIDS determined by a quantitative antigen enzyme immunoassay. Clin Vaccine Immunol. 2011 Apr;18(4):661-6. doi: 10.1128/CVI.00389-10. Epub 2011 Feb 9. PMID: 21307278; PMCID: PMC3122570.
- Wheat LJ, Connolly P, Haddad N, Le Monte A, Brizendine E, Hafner R. Antigen clearance during treatment of disseminated histoplasmosis with itraconazole versus fluconazole in patients with AIDS. Antimicrob Agents Chemother. 2002;46(1):248-250. doi:10.1128/aac.46.1.248-250.2002
- Wheat LJ, Cloud G, Johnson PC, Connolly P, Goldman M, Le Monte A, Fuller DE, Davis TE, Hafner R; AIDS Clinical Trials Group; Mycoses Study Group of NIAID. Clearance of fungal burden during treatment of disseminated histoplasmosis with liposomal amphotericin B versus itraconazole. Antimicrob Agents Chemother. 2001 Aug;45(8):2354-7. doi: 10.1128/AAC.45.8.2354-2357.2001. PMID: 11451696; PMCID: PMC90653.
- Wheat LJ, Connolly-Stringfield P, Kohler RB, Frame PT, Gupta MR. Histoplasma capsulatum polysaccharide antigen detection in diagnosis and management of disseminated histoplasmosis in patients with acquired immunodeficiency syndrome. Am J Med. 1989 Oct;87(4):396-400. doi: 10.1016/s0002-9343(89)80820-4. PMID: 2801730.
- Wheat LJ, Connolly-Stringfield P, Blair R, Connolly K, Garringer T, Katz BP. Histoplasmosis relapse in patients with AIDS: detection using Histoplasma capsulatum variety capsulatum antigen levels. Ann Intern Med. 1991 Dec 15;115(12):936-41. doi: 10.7326/0003-4819-115-12-936. PMID: 1952490.
- Henry Masur, John T. Brooks, Constance A. Benson, King K. Holmes, Alice K. Pau, Jonathan E. Kaplan, Prevention and Treatment of Opportunistic Infections in HIV-Infected Adults and Adolescents: Updated Guidelines From the Centers for Disease Control and Prevention, National Institutes of Health, and HIV Medicine Association of the Infectious Diseases Society of America, Clinical Infectious Diseases, Volume 58, Issue 9, 1 May 2014, Pages 1308–1311, https://doi.org/10.1093/cid/ciu094