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Abstract
Objective. The purpose of this investigation was to improve a rankit ordinal model for evaluating and validating dichotomized tests in a prospective Nordic project. Material and methods. The model is based on the assumption that the S‐shaped curve of fractions of positive for increasing concentrations can be de‐convoluted to a histogram and thereby used to calculate the parameters for a ln‐Gaussian distribution. In a Nordic survey, four urine samples with known concentrations of hCG (human chorionic gonadotrophin) and nitrites were distributed to more than 2500 practitioners' offices. Results. The results are presented as parameters (geometric mean and CV) for the components urine‐hCG and urine‐nitrites, together with fractions of positive for clinical critical values (5 and 40 IU/L for hCG), for which fractions should be below 0.01 and above 0.99, respectively, and 7 µmol/L for nitrites. Furthermore, the concentration intervals of varying fractions of positive from 0.01 to 0.99 are estimated as grey zones. The parameters and grey zones for different kits are compared. No urine‐hCG kit fulfilled the low clinical criterion, whereas all fulfilled the high criterion. Seven of the eight nitrites kits had fractions of positive above 0.9 for the company confirmation limit, but varying fractions for the clinically important limit of 7 µmol/L (fractions from 0.06 to 0.83). Conclusions. The present model makes it easy to estimate parameters for the kits, and also to estimate the fractions of measured positives for specified concentrations. It is thus suited for external quality assessment as well as for manufacturers' method validation.
| hCG | = | human Chorionic Gonadorophin |
| IFCC | = | International Federation of Clinical Chemistry |
| MEQUALAN | = | Metrology of qualitative chemical analysis (under the European Commission) |
| = | μ = Mean of ln‐values | |
| = | GM = Geometric Mean = eμ | |
| = | σ = Standard deviation of ln‐values | |
| = | CV = Coefficient of variation of natural values ˜ σ (CV % ˜ σx100) | |
| CI | = | Confidence interval |
| Xi | = | Midpoint of interval of the hypothetical histogram |
| fi | = | Relative frequency of the hypothetical histogram |
| EQA | = | External Quality Assurance |
| NOKLUS | = | Norwegian quality improvement of primary care laboratories, Division for General Practice, Norway |
| EQUALIS | = | External Quality Assurance in Laboratory Medicine in Sweden, Uppsala, Sweden |
| DEKS | = | Danish Institute for External Quality Assurance for Laboratories in Health Care, Herlev, Denmark |
| LABQUALITY | = | External quality assurance institute, Helsinki, Finland |
| = | Limit of detection, LD = percentage positive at a defined low concentration level (4) | |
| = | Confirmation limit, LC = percentage negative at a defined high concentration level (4) |
| hCG | = | human Chorionic Gonadorophin |
| IFCC | = | International Federation of Clinical Chemistry |
| MEQUALAN | = | Metrology of qualitative chemical analysis (under the European Commission) |
| = | μ = Mean of ln‐values | |
| = | GM = Geometric Mean = eμ | |
| = | σ = Standard deviation of ln‐values | |
| = | CV = Coefficient of variation of natural values ˜ σ (CV % ˜ σx100) | |
| CI | = | Confidence interval |
| Xi | = | Midpoint of interval of the hypothetical histogram |
| fi | = | Relative frequency of the hypothetical histogram |
| EQA | = | External Quality Assurance |
| NOKLUS | = | Norwegian quality improvement of primary care laboratories, Division for General Practice, Norway |
| EQUALIS | = | External Quality Assurance in Laboratory Medicine in Sweden, Uppsala, Sweden |
| DEKS | = | Danish Institute for External Quality Assurance for Laboratories in Health Care, Herlev, Denmark |
| LABQUALITY | = | External quality assurance institute, Helsinki, Finland |
| = | Limit of detection, LD = percentage positive at a defined low concentration level (4) | |
| = | Confirmation limit, LC = percentage negative at a defined high concentration level (4) |
Acknowledgements
We thank NORDFOND for economic support in arranging two project meetings, and the people in the participating Nordic control organizations as well as the many participants in the two surveys.
We are grateful to the Nordic EQA organizers NOKLUS, EQUALIS, LABQUALITY, DEKS, NKK, IKK and to LKO for help in distributing control samples and collecting data, as well as for critical advice during preparation of the manuscript. We also thank Michael J. Pugia (Siemens), Reinhard Moeller (Quidel Corporation) and Pernille Schmidt (Roche) for valuable input in relation to interpretation of the many terms used for measurements on the ordinal scale.