title: How many people are living with undiagnosed HIV infection? An estimate for Italy, based on surveillance data.
abstract: OBJECTIVE
  To estimate the size and characteristics of the undiagnosed HIV population in Italy in 2012 applying a method that does not require surveillance data from the beginning of the HIV epidemic.
  
  
  METHODS
  We adapted the method known as 'London method 2'; the undiagnosed population is estimated as the ratio between the estimated annual number of simultaneous HIV/clinical AIDS diagnoses and the expected annual progression rate to clinical AIDS in the undiagnosed HIV population; the latter is estimated using the CD4⁺ cell count distribution of asymptomatic patients reported to surveillance. Under-reporting/ascertainment of new diagnoses was also considered. Also, the total number of people living with HIV was estimated.
  
  
  RESULTS
  The undiagnosed HIV population in 2012 was 13,729 (95% confidence interval: 12,152-15,592), 15,102 (13,366-17,151) and 16,475 (14,581-18,710), assuming no under-reporting/ascertainment, 10 and 20% of under-reporting/ascertainment, respectively. The percentage of undiagnosed cases was higher among HIV people aged below 25 years (25-28%), MSM (16-19%) and people born abroad (16-19%), whereas it was small among injection drug users (3%).
  
  
  CONCLUSIONS
  The estimate of people in Italy with undiagnosed HIV in 2012 was in a plausible range of 12,000-18,000 cases, corresponding to 11-13% of the overall prevalence. The method is straightforward to implement only requiring annual information from the HIV surveillance system about CD4⁺ cell count and clinical stage at HIV diagnosis. Thus, it could be used to monitor if a certain prevention initiative lead to the reduction of the undiagnosed HIV population over time. It can also be easily implemented in other countries collecting the same basic information from the HIV surveillance system.
authors.affiliation: a'L. Spallanzani' National Institute for Infectious Diseases, IRCCS bIstituto Superiore di Sanità, Rome, Italy.
authors.forename: Alessia
authors.initials: A
authors.lastname: Mammone
authors.forename: Patrizio
authors.initials: P
authors.lastname: Pezzotti
authors.forename: Vincenza
authors.initials: V
authors.lastname: Regine
authors.forename: Laura
authors.initials: L
authors.lastname: Camoni
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Puro
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Ippolito
authors.forename: Barbara
authors.initials: B
authors.lastname: Suligoi
authors.forename: Enrico
authors.initials: E
authors.lastname: Girardi
mesh_terms: D000293:Adolescent; D000328:Adult; D018563:Disease Notification; D062665:Epidemiological Monitoring; D005260:Female; D015658:HIV Infections / Q000175:diagnosis* / Q000453:epidemiology*; D006801:Humans; D007558:Italy / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D055815:Young Adult