Evaluation of the Correlation Coefficient of Polyethylene Glycol Treated and Direct Prolactin Results and Comparability with Different Assay System Results

The presence of Macro prolactin is a significant cause of elevated prolactin resulting in misdiagnosis in all automated systems. Poly ethylene glycol (PEG) pretreatment is the preventive process but such process includes the probability of loss of a fraction of bioactive prolactin. Surprisingly, PEG treated EQAS & IQAS samples in Cobas e 411 are found out to be correlating with direct results of at least 3 immunoassay systems and treated and untreated Cobas e 411 results are comparable by a correlation coefficient. Comparison of EQAS, IQAS and patient samples were done to find out the trueness of such correlation factor. Study with patient’s results have established the correlation coefficient is valid for very small concentration of prolactin also. MATERIALS AND METHODS: EQAS, IQAS and 150 patient samples were treated with PEG and prolactin results of treated and untreated samples obtained from Roche Cobas e 411. 25 patient’s results (treated) were compared with direct results in Advia Centaur, Architect I & Access2 systems. STATISTICAL CALCULATIONS: Correlation coefficient was obtained from trend line of the treated and untreated results. Two tailed p-value obtained from regression coefficient(r) and sample size. RESULTS AND DISCUSSION: The correlation coefficient is in the range (0.761-0.771). Reverse correlation range is (1.289-1.301). r value of two sets of calculated results were 0.995. Two tailed p- value is zero approving dismissal of null hypothesis. CONCLUSION: The z-score of EQAS does not always assure authenticity of results. PEG precipitation is correlated by the factor 0.761 even in very small concentrations. Abbreviations GFC: gel filtration chromatography PEG: polyethylene glycol EQAS: external quality assurance system M-PRL: macro prolactin PRL: prolactin ECLIA: electro-chemiluminescence immunoassay CLIA: clinical laboratory improvement amendments IQAS: internal quality assurance system r: regression coefficient