Stabilizing specimens for routine ammonia testing in the clinical laboratory.

BACKGROUND

In vitro deamination generates ammonia in freshly collected blood specimens. To prevent this, samples for ammonia testing are usually collected on ice and run rapidly (e.g., within 1h). We developed a method to stabilize specimens for ammonia analysis.

METHODS

Following plasma separation, 500μmol/l cycloserine or a combination of 2mmol/l sodium borate with 5mmol/l l-serine were added to sample pools with normal or increased concentrations of ALT and/or GGT to inhibit deamination; and/or residual platelets were removed via centrifugation. Sample pools were then incubated at room temperature or 4°C. Untreated sample pools were also incubated at -80°C. Ammonia was measured at 0, 1, 2, 4, 8, 16, and 24h.

RESULTS

When incubated at 4°C without treatment, sample pools with enzymes within their reference limit had an increase of 0.5μmol/l/h, whereas sample pools with ALT and/or GGT activity above their upper reference limit had an increase of 3.6μmol/l/h (p<0.001). When sample pools were incubated at 4°C with sodium borate/l-serine, the rate of ammonia increase was significantly reduced in samples with normal (0.3μmol/l/h, p<0.001 vs. untreated controls) or high enzyme activity (0.1μmol/l/h, p<0.001 vs. untreated controls). Independent of the ALT and/or GGT concentrations, storing the sample at -80°C also preserved the specimens for ammonia analysis (0.2μmol/l/h, p<0.001 vs. untreated controls).

CONCLUSIONS

By combining sodium borate/l-serine with refrigeration, plasma ammonia specimens can be stabilized for >12h.