infant respiratory distress syndrome ( irds ) is the most common respiratory disease in preterm infants , and leads to substantial morbidity and mortality.1,2 irds is caused by lung immaturity and usually develops within minutes of birth . it is defined by tachypnea , retractions or nasal flaring , grunting respiration , and possibly central cyanosis.3 it occurs in approximately 0.3%1.2% of live - born infants.46 however , the prevalence of irds increases with decreasing gestational age.4,7 previous studies have found a prevalence of approximately 90% in premature infants born in gestational week 28.8 valid data on irds are important for clinical epidemiological research . if the coding is accurate , the danish medical registries provide excellent data to study the long - term prognosis of irds , as the registries comprise more than 30 years of medical observations.9,10 to our knowledge , no study has examined the validity of the irds diagnosis in administrative registries . we therefore conducted the present study with the objective of estimating the positive predictive value ( ppv ) of the irds diagnosis recorded in the population - based danish national patient registry ( dnpr ) according to the international classification of diseases ( icd ) , 8th and 10th revisions , using medical records as reference standard . based on the dnpr , we identified patients diagnosed with irds from january 1 , 1977 to december 31 , 2008 in the northern part of denmark ( corresponding to the former north jutland county ) . this part of denmark has approximately 500,000 inhabitants , equivalent to approximately 11% of the total danish population . the dnpr includes data on all non - psychiatric hospital admissions in the country since 1977 and outpatient clinic and emergency room visits since 1995 . data include the patients civil registration number , which is a unique personal identification number assigned to all danish residents , date of admission and discharge , surgical procedure(s ) performed , one primary diagnosis and up to 19 secondary diagnoses coded by the discharging physician according to the icd-8 until the end of 1993 and subsequently the icd-10 . the primary diagnosis code registered is the main reason for the hospital contact . among all patients with a primary or secondary irds diagnosis , we randomly selected three irds patients for each calendar year , 96 in total , between 1977 and 2008 . the irds hospital admissions were identified based on the icd-8 diagnosis code 776.19 ( idiopathic respiratory distress syndrome or hyaline membrane disease ) and the icd-10 diagnosis code p22.0 ( idiopathic respiratory distress syndrome ) . based on the dnpr , we identified patients diagnosed with irds from january 1 , 1977 to december 31 , 2008 in the northern part of denmark ( corresponding to the former north jutland county ) . this part of denmark has approximately 500,000 inhabitants , equivalent to approximately 11% of the total danish population . the dnpr includes data on all non - psychiatric hospital admissions in the country since 1977 and outpatient clinic and emergency room visits since 1995 . data include the patients civil registration number , which is a unique personal identification number assigned to all danish residents , date of admission and discharge , surgical procedure(s ) performed , one primary diagnosis and up to 19 secondary diagnoses coded by the discharging physician according to the icd-8 until the end of 1993 and subsequently the icd-10 . the primary diagnosis code registered is the main reason for the hospital contact . among all patients with a primary or secondary irds diagnosis , we randomly selected three irds patients for each calendar year , 96 in total , between 1977 and 2008 . the irds hospital admissions were identified based on the icd-8 diagnosis code 776.19 ( idiopathic respiratory distress syndrome or hyaline membrane disease ) and the icd-10 diagnosis code p22.0 ( idiopathic respiratory distress syndrome ) . the medical records of the identified irds patients were reviewed and data were entered in epidata ( epidata association , odense , denmark ) by a physician ( skt ) . where there was doubt with regard to interpretation of the medical record , another physician ( cfc ) we also noted gender , gestational age , treatment with continuous positive airway pressure ( cpap ) , and whether the irds diagnosis was mentioned explicitly in the medical record . in the primary analysis , we defined irds as the presence of at least two of the four clinical symptoms ( tachypnea , retractions or nasal flaring , grunting , and central cyanosis ) , which had to be present for more than 30 minutes . tachypnea was defined as 60 or more breaths per minute . in an additional analysis , we defined irds as two or more clinical symptoms together with a positive x - ray finding , defined as reticulogranular ground - glass appearance with air bronchograms . if no information was available on whether or not an x - ray had been taken , or if the radiologist had explicitly ruled out signs of irds , we classified the individual as not having irds . for descriptive purposes , we abstracted data on cpap treatment , but only if provided by pediatric departments and for more than 30 minutes . furthermore , we noted if the irds diagnosis was mentioned in the medical record as a confirmed diagnosis . we used the medical records as the reference standard when computing the ppv of the irds diagnosis . the ppv was defined as the proportion of patients registered with an irds diagnosis in the dnpr that was confirmed by medical record review . thus , the numerator was the number of confirmed irds cases according to the medical records , and the denominator was the selected number of patients registered with an irds diagnosis in the dnpr . the 95% confidence intervals ( cis ) were computed using jeffrey s method.11 we stratified the analyses by primary and secondary diagnoses , by icd-8 ( 19771993 ) and icd-10 ( 19942008 ) periods , and by gestational age ( gestational week < 28 , 2831 , 3236 , and 37 ) . for 90 of the 96 ( 94% ) selected patients with an irds diagnosis in the dnpr , we were able to find the corresponding medical record , of which 52 ( 58% ) were for males and 38 ( 42% ) were for females . gestational age was reported in the medical record for 88 irds patients , of whom 65 ( 74% ) were preterm infants , and 23 ( 26% ) were born at term ( 37 weeks of gestation or later ) . from the medical record we were able to confirm 73 of the 90 patients coded with an irds diagnosis . this gave us an overall ppv of 81% ( 95% ci 72%88% ) ( table 1 ) . in the additional analysis , with irds defined as two or more clinical symptoms of irds and a confirmed x - ray ( 52 irds patients ) , the ppv was 58% ( 95% ci 48%68% ) . irds was registered in the dnpr as the primary diagnosis for 20 ( 22% ) patients , of which 14 were confirmed by the medical record review , corresponding to a ppv of 70% ( 95% ci 48%86% ) . among 70 ( 78% ) patients registered with irds as a secondary diagnosis , 59 were confirmed irds patients , corresponding to a ppv of 84% ( 95% ci 75%91% ) . we found a ppv of 87% ( 95% ci 75%94% ) in the icd-8 period ( 19771993 ) and a ppv of 75% ( 95% ci 61%86% ) in the icd-10 period ( 19942008 ) . the ppv among males was 75% ( 95% ci 62%85% ) , while it was 89% ( 95% ci 77%96% ) among females . stratified by gestational age , we found a ppv ranging from 61% ( 95% ci 41%79% ) in infants born at 37 weeks of gestation or later , to 92% ( 95% ci 77%98% ) in infants born between 28 and 31 weeks of gestation ( table 1 ) . the irds diagnosis was explicitly mentioned in 71 ( 79% ) of the 90 medical records , and 82 ( 91% ) of the patients with an irds diagnosis were treated with cpap for more than 30 minutes . in this study , we found reasonable accuracy of the coding for irds in the dnpr as confirmed by exact description of the symptoms of irds in the medical record . to our knowledge , this is the first study to examine the validity of the dnpr with regards to irds . other studies have estimated the ppv of other neonatal diagnoses in the dnpr , including the diagnoses of congenital cardiac malformations with overlapping time periods , 19942002 and 20002008.12,13 they found overall ppvs of 89% ( 95% ci 86%92% ) and 90% ( 95% ci 89%91%),12,13 which is slightly higher than our ppv , probably because irds is a syndrome characterized by the co - occurrence of characteristic symptoms . we found a slightly lower ppv in the icd-10 period than in the icd-8 period . the potential decrease in ppv over time may reflect less optimal coding practices in the later period , but may also be explained by better documentation of symptoms in the medical records of the early period . we defined the irds diagnosis as the presence of a minimum of two of four clinical symptoms in the medical record . however , these symptoms may not always be described in the medical records of irds patients . if conditions such as infections or congenital heart disease were overlooked by the clinicians , we may have overestimated the ppv of the irds diagnosis . we did not include an x - ray finding of irds in our main criteria , because x - rays were not routinely performed in patients with mild irds . as expected , we found a higher ppv of the irds diagnosis among infants born preterm than among infants born at term . we only examined one region in denmark ; however , we find it reasonable to believe that the results are representative for the entire country owing to the uniform danish health care system . further , we were not able to report on sensitivity , ie , the proportion of all patients with irds actually registered in the dnpr , as we only included patients with a dnpr diagnosis of irds . however , the completeness of the dnpr has previously been estimated to be approximately 90%.12,14 further , we were not able to blind the irds diagnosis for the physician who reviewed the medical records ; however , this is unlikely to have had any major influence on our findings . the ppv of the irds diagnosis quantified in our study may be applied in sensitivity analyses in future studies , to examine the potential effect of the misclassification on study results . we found a reasonable ppv of 81% ( 95% ci 72%88% ) of the irds diagnosis in the dnpr , when compared with symptoms described in the infants medical record . the dnpr is a useful data source for studies of irds , particularly if restricted to preterm infants .