were also asked to return all empty vials of GH to the growth hormone administrator each month, using a prepaid courier bag sent directly to the family (GHret). Parents were actively encouraged, but could not be mandated to return all empty vials. To ensure that no empty GH vials were left over from periods preceding the study, empty vials were requested starting the month before the study. Compliance was evaluated in two ways: number of GH vials required each month (GHreq; based on verbal self-report by the patient’s caregivers) and number of used GH vials returned (GHret), with both expressed as a percentage of the number of vials prescribed for each patient. Based on the known doses of GH prescribed, no patients should have used less than one vial of GH over the 4-month duration of this investigation, and therefore patients who returned no vials over the study period were excluded from analysis. Growth data are routinely obtained every 6 months for all patients. Growth rates were calculated for the 6–8 month-period that encompassed the interval of this audit. Changes in height were recorded and converted to height velocity standard deviation scores (HVSDS) to control for sex and age differences. For the purposes of analysis, compliance was defined as 85% or better adherence to the prescribed treatment, equivalent to an average of one missed dose a week. Patients were divided into three groups according to their rate of compliance: High compliance (missed #1 dose a week on average), Medium (missed .1 and ,3 doses/ week), and Low (missed $3 doses/week). The association between GHret and GHreq to HVSDS was assessed using non-parametric Spearman’s rank correlations. HVSDS was compared across compliance groups using a general linear regression model (GLM), which also included as factors diagnosis (i.e. KIGS code 1, 2 or 3) and an interaction between diagnosis and compliance group, in order to account for differential growth responses to GH treatment. Subsequently, the effects of age, sex, ethnicity, area of residence (cities (.100,000 inhabitants) vs. towns and rural areas) and diagnosis on GH compliance for each subject were examined. A GLM was used with compliance rate as the response, and sex, ethnicity, diagnosis and area of residence as independent variables, and age included as covariate. A binary logistic linear regression was then adopted to determine if any of the above factors had an effect on whether a patient was compliant or not. Posthoc pairwise comparisons were carried out using Tukey’s simultaneous tests. All analyses were carried out in Minitab (Minitab v.15, Pennsylvania State University, USA), with Johnson transformation as required to stabilize the variance. Data are presented as mean 6 SEM. Results 175 patients received GH in the 4 month interval of the analysis and were 12.160.6 years old with 48% males. Diagnoses included GH deficiency (57%), Turner Syndrome (27%), idiopathic short stature (7%), small for gestational age (6%), Prader Willi Syndrome (3%) and other (6%). The caregivers identified the children as European & other (75%), Maori (15%), Asian (7%), and Pacific Islander (4%). 35% of patients resided in Auckland, 34% elsewhere in the North Island, and 22% in the South Island. A total of 25 patients failed to return a single vial during the study period, and were excluded from the GHret analysis. The overall estimated rate of non-compliance (i.e. patients missing more than one injection per week) was 66% (73/110) according to GHret and 34% (59/172) according to GHreq. HVSDS was positively correlated with the rate of returned vials (r = 0.20; p,0.05) but not with GHreq. Further, average linear growth of Compliance with GH Treatment in New Zealand patients increased as GHret compliance levels increased, and missing more than one dose a week significantly reduced linear growth (p,0.01) (Fig. 1), whereas GHreq showed no association with growth (p = 0.83). GHret was therefore used for subsequent analyses. Finally, patients with organic GH-deficiency tended to grow faster than subjects with other causes of short stature (HVSDS 2.3260.60 vs 0.9360.38; p,0.06). Compliance rates were not associated with age, gender, diagnosis or area of residence. However, Maori patients missed nearly one extra GH dose a week on average in comparison to other ethnic groups (p,0.05). Discussion This national survey indicates that non-compliance with GH treatment is very common in New Zealand, based on objective data, in a setting where there is regular, direct contact with caregivers and no personal financial constraints on GH uptake. Further, the two thirds of patients who on average missed one or more doses of GH each week showed significantly reduced linear growth compared with compliant patients. To our knowledge our study is the first to provide robust evidence that non-compliance is a common cause of suboptimal response to GH treatment in children and adolescents. The use of returned GH vials in this study provided an objective measure of compliance in a complete population of children and adolescents receiving GH. A higher GHret should reflect the number of vials used in the previous month and thus denote greater compliance. A potential limitation is that return of used GH vials was not a mandatory pre-condition to receive new vials. Therefore, although GHret was an objective measure in contrast with caregiver-reported GHreq, vials might have been broken, lost, or the empty vials simply not sent back to the programme, so that GHret could have underestimated compliance. Nonetheless, the strong correlation with patient growth suggests that this approach still provided a reasonably valid measure of compliance. An alternative approach would be the use of electronic injection devices with a dosage counter to accurately assess patient compliance. Such devices are available but are limited by cost, complexity and robustness. This may become practical for Figure 1. Height velocity standard deviation scores (HVSDS) over 6–8 months according to the level of compliance with GH treatment: High (n = 30) missed #1 dose/week, Medium (n = 51) missed .1 and ,3 doses/week, and Low (n = 29) missed $3 doses/week. Data are mean 6 SEM. **p,0.01, ***p,0.001 vs High. doi:10.1371/journal.pone.0016223.g001 PLoS ONE | www.plosone.org 2 January 2011 | Volume 6 | Issue 1 | e16223