Sapphire 400: low creatinine results with frequent recalibration
What happened
On a Sapphire 400 chemistry analyzer, creatinine results ran consistently low and the instrument needed recalibration every 2-3 days, as reported in the case.
What was checked
- The engineers checked reagent stability: the alkaline NaOH-based creatinine reagent absorbs CO2 from the air, its pH drifts down, and the Jaffe reaction rate with picric acid drops with it.
- They also reviewed how staff handled the reagent and found it was being topped up to the old stock instead of preparing a fresh portion.
Cause
Degraded creatinine reagent: absorbed CO2 lowered the pH of the alkaline reagent and slowed the Jaffe reaction, while repeated topping-up kept partially spent reagent in use.
What was done
The fix was procedural, no parts involved: work in the two-reagent (bireagent) setup instead of pre-mixing a single reagent, keep the on-board stock small — reported in the case as no more than 5-7 days, preferably 1-2 days, discard the remainder when QC fails, run a reagent blank every 24-48 hours, and always prepare a fresh portion rather than topping up. With this practice the calibration interval in the case lengthened to the reported 10-15 days and the low bias was resolved.
Safety
Treat patient samples and any reagent spill as potentially infectious (gloves, eye protection), and note that the creatinine reagent is strongly alkaline — avoid skin contact and follow the laboratory's waste-disposal rules.
Lesson: Open-vial alkaline chemistry reagents quietly lose strength to CO2 uptake and topping-up, so small on-board volumes, scheduled blanks and fresh preparation protect result quality better than tightening the calibration routine.
Covered in the course: Lab Analyser Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.