OBJECTIVE: To examine the merits of measuring free analytes by ultrafiltration using either diluted or undiluted serum. RESULTS: Confidence in the accuracy of measurements is affected both by problems identified in current systems using semipermeable membranes, the sensitivity of the system to artefacts and comparisons with other imperfect assays. CONCLUSION: All "gold standard" methods must robustly obey sound physicochemical principles if valid conclusions are to be drawn.
OBJECTIVE: To examine the merits of measuring free analytes by ultrafiltration using either diluted or undiluted serum. RESULTS: Confidence in the accuracy of measurements is affected both by problems identified in current systems using semipermeable membranes, the sensitivity of the system to artefacts and comparisons with other imperfect assays. CONCLUSION: All "gold standard" methods must robustly obey sound physicochemical principles if valid conclusions are to be drawn.