A total of 342 (10%) out of 3,478 households with children aged 17 years old or younger reported that 1 or more children in the household had a serious or chronic health condition. Out of 6,773 children in these households, 4 were reported to have an immunodeficiency disease. However, in response to a follow-up question about the specific diagnosis, one of these cases had been diagnosed with ITP, an autoimmune disorder. The other three children were reported to have diagnoses that are PID: agammaglobulinemia, selective IgA deficiency, and IgG subclass deficiency.
All household informants were then asked: “Has anyone in your household ever been diagnosed with a primary immunodeficiency disease, such as common variable immunodeficiency, IgA deficiency, IgG subclass deficiency or any other immunodeficiency? (This is not acquired immunodeficiency—AIDS).” In 131 (1.3%) out of 10,005 households surveyed, the household informant reported that someone in the household had been diagnosed with a PID (Fig. 1). However, when asked the specific diagnosis for the condition, most (66%) reported a diagnosis other than a primary immunodeficiency (Table 1). In many of these cases, the diagnosed condition was either an autoimmune disease or one that had an immunological component. Half of the remaining cases either had no diagnosis given (16%) or the condition could not be classified from the description (3%) (Fig. 2).
Nonetheless, a total of 23 household members in 18 households were reported with a specific diagnosis for PID (CVID, IgA, IgG, XLA, SCID, CGD) (Fig. 3). These findings suggest a population prevalence of diagnosed PID at 1 in 1,200 persons in the United States. This translates into a population rate for diagnosed PID of 0.0863%. When applied to the U.S. population of 297,386,000 persons, this gives an estimate of approximately 250,000 persons with diagnosed PID in the United States. The confidence bounds about this estimate at the 95% confidence level suggest that the number of persons with the diagnoses of PID in the United States will be between 152,000 and 361,000 (Table 2).
The population characteristics of this national probability sample of persons diagnosed with PID can be compared to two earlier large scale, nationally distributed but nonprobability surveys of patients conducted by the Immune Deficiency Foundation in 1996–1997 (N = 3,046) and 2002 (N = 1,526). The first IDF survey of patients was distributed through physicians and medical centers known to the Foundation to be treating patients (First Patient Survey) . The second IDF survey of patients was mailed to patients on IDF’s mailing list who had not participated in the first survey (Second Patient Survey) .
The proportion of males in the national probability sample (56%) is somewhat higher than in the first IDF survey (48%) and the second IDF survey (42%). The proportion of patients under 18 years of age in the national probability sample (43%) falls between the proportion under 18 years of age in the first IDF survey (40%) and the second IDF survey (50%) (Fig. 4). In general, the age and gender distribution of the probability sample is consistent with the characteristics of patients known to the foundation.
The current Clinical Care Guidelines for Patients with Primary Immunodeficiency Diseases recommend treatment with intravenous immunoglobulin (IVIG) for agammaglobulinemia, common variable immunodeficiency, hyper IgM, Wiskott–Aldrich, and severe combined immunodeficiency. Specific antibody deficiency, which may be a part of IgG subclass deficiency, is also indicated although IgG subclass deficiency alone is not. The majority of the patients in the probability sample (57%) have diagnoses for which IVIG therapy is the standard of care. This is similar to the proportion of patients in the second IDF survey with diagnoses for which IVIG is the standard of care (65%). However, only 22% of PID patients in the national household telephone survey sample were currently being treated with IVIG compared to 67% of PID patients in the second IDF survey. Indeed, only 67% of patients with aggammaglobulinemia and 13% of patients with common variable immunodeficiency in the national probability sample were currently being treated with IVIG (Fig. 5). Hence, the surveys suggest that immune deficient patients known to the IDF, either through referral by specialist or individual information-seeking are more likely to be following the recommended therapy for the condition than the average patient in the general community.
The national telephone survey confirmed that, despite an IDF mailing list of more than 10,000 diagnosed PID patients, only 4 out of 18 households containing a total of 23 PID patients in a national probability sample had ever heard of the Foundation, and only 1 out of 18 received the IDF newsletter. If the rate of 1 out of 18 households with a patient receiving a newsletter is divided into the IDF newsletter mailing list of 10,000, it yields an independent estimate of 180,000 households with patients diagnosed with a PID. If this household estimate is converted to a population estimate of 1.3 patients per household (23/18), then the survey yields an independent national estimate of approximately 230,000 persons diagnosed with a PID.