Do Serum Creatinine Levels Show Clinically Significant Fluctuations on Serial Determinations on the Siemens Advia 1800 Analyzer?

BACKGROUND: The goal of this work was to determine whether there are clinically significant fluctuations in the level of serum creatinine on serial determinations, especially in the borderline range (1.1-1.3 mg/dl), after specimen storage. METHODS: Sixty-one serum samples were analyzed. They were divided into three categories based on the initial serum creatinine measurement: low (≤1.0 mg/dl), borderline (1.1-1.3 mg/dl), and high (≥1.4 mg/dl). The specimens were stored at 4°C and run on the Siemens Advia 1800 chemistry analyzer on days 1, 3, and 11. RESULTS: Statistical comparisons of the three groups were made using the unpaired t-test, yielding a two-tailed P-value for each group comparison. The P-values ranged from 0.0829 to 0.3892, indicating no statistically significant difference between the standard deviations of each group. CONCLUSIONS: Mild-to-moderate fluctuations in precision occur in successive serum creatinine determinations. The overwhelming majority of these fluctuations should not affect clinical decision making.

Reproducibility of Serum Potassium Values in Serum From Blood Samples Stored for Increasing Times Prior to Centrifugation and Analysis.

GOALS: The goal of this work was to determine if immediate versus postponed centrifugation of samples affects the levels of serum potassium. METHODS: Twenty participants donated normal venous blood that was collected in four serum separator tubes per donor, each of which was analyzed at 0, 1, 2, or 4 hr on the Siemens Advia 1800 autoanalyzer. RESULTS: Coefficients of variation (CVs) for potassium levels ranged from 0% to 7.6% with a mean of 3 ± 2%. ANOVA testing of the means for all 20 samples showed a P-value of 0.72 (>0.05) indicating that there was no statistically significant difference between the means of the samples at the four time points. Sixteen samples were found to have CVs that were ≤5%. Two samples showed increases of potassium from the reference range to levels higher than the upper reference limit, one of which had a 4-hr value that was within the reference or normal range (3.5-5 mEq/l). Overall, most samples were found to have reproducible levels of serum potassium. CONCLUSIONS: Serum potassium levels from stored whole blood collected in serum separator tubes are, for the most part, stable at room temperature for at least 4 hr prior to analysis. However, some samples can exhibit significant fluctuations of values.

Stability of BUN and creatinine determinations on the Siemens Advia 1800 analyzer.

BACKGROUND: Serum creatinine values of patients tend to change as a result of the use of different blanks used for creatinine determinations on the Advia 1650. After upgrading the analyzer to the Advia 1800, creatinine values tended to be more reproducible. As part of a quality assurance investigation to test the reproducibilities of creatinine values, we determined serial creatinine values in the sera of 13 patients whose initial values were either in the reference range or elevated (range 0.58-7.8 mg/dl). These values were determined concurrently with serum blood urea nitrogen (BUN) determinations (range 6.0-84.4 mg/dl) as these two analytes are used together in evaluation of renal function. METHODS: We determined BUN and creatinine values, using the glutamate dehydrogenase lined enzyme assay system and the Jaffe method, respectively. RESULTS: We find that all values for creatinine on samples stored at 4 °C were reproducible as were the corresponding BUN values, which is revealed by low values for the coefficients of variation (CVs), that is, mean CV of 4.55% for creatinine and 2.52% for BUN. One sample with relatively high CV (10.6%) for creatinine was found to have an initial value of 1.1 mg/dl, in the reference range; but, on repeat determinations, the obtained levels were as high as 1.5 mg/dl, above the reference range. BUN values for this sample remained in the reference range, suggesting that no renal disease was present. CONCLUSION: We conclude that creatinine and BUN determinations are stable, but occasional spurious creatinine values can occur on the Advia 1800 analyzer.

Comparison of glucose determinations on blood samples collected in three types of tubes.

Because of the metabolism of serum glucose in collection tubes containing blood samples, serum glucose levels may be found to decrease over time. Several types of collection tubes have been designed to, at least partially, block glucose metabolism by red blood cells in blood collection tubes that may not be analyzed immediately after blood collection. These include red-top collection tubes with serum separator, grey-top tubes with a fluoride glycolysis inhibitor, and heparin-containing green-top tubes which prevent clot formation. As part of a quality assurance project, we investigated whether glucose levels differed in the three tube types from each of 18 volunteers on a prolonged standing of 4 hours. We then determined the glucose concentrations of all three tubes from each of the 18 volunteers. We used refrigerated samples over a five-day period to determine if the initial values were reproducible. Surprisingly, after standing for four hours at room temperature, we found that the glucose levels in the three tubes from each volunteer were statistically indistinguishable from one another using the two-tailed paired t-test. Also, a linear regression analysis showed that the values of glucose for the three pairs of two tube types were closely correlated with one another, with correlation coefficients of >0.97, slopes close to 1, and Y-intercepts close to 0. These results suggest that blood collection in any of these tubes will render similar values for serum glucose even after standing for four hours. The tubes were then refrigerated at 4°C and re-analyzed after another six hours and then once per day for the next four days. Beginning at the first day at the six-hour determination, the glucose levels in the red- and grey-top tubes were statistically indistinguishable from one another but not in the red- and green-top tubes and in the grey- and green-top tubes. This was due to a steady decrease in the glucose levels in the green-top tubes. The glucose levels in the red- and grey-top tubes from each volunteer remained constant over the five-day period so that the coefficients of variation (CV) were low. In contrast, due to the decrease of glucose levels in the green-top tubes, the CVs for repeated glucose determinations in these tubes were high. Interestingly, a regression analysis of the glucose values for all three sets of paired tubes showed high (> 0.97) correlation coefficients and slopes close to 1. However, a regression analysis of the glucose values in the red- and green-top and grey- and green-top tubes at day five showed Y-intercepts of about -32 suggesting that there is a constant decrease of glucose in the green-top tubes that amounts to approximately 6 mg/dL per day over five days. These results suggest that red-top tubes with serum separator or grey-top tubes with a fluoride glycolysis inhibitor may be used for reproducible glucose determinations.