Unexpected Hypoglycemia on an In-House Chemistry Panel
A 5-year-old indoor domestic shorthair cat is presented for a wellness examination. The cat is bright, alert, and eating normal food; the owner reports no vomiting, seizures, or weakness. Blood is collected from the jugular vein into a serum separator tube at 9:15 a.m. and left on the counter because the analyzer is occupied. At 10:40 a.m. the sample is centrifuged and run. Results include glucose 42 mg/dL (reference 70-150), BUN 22 mg/dL, creatinine 1.4 mg/dL, and total protein 7.8 g/dL. The PCV is 38% and the blood smear is unremarkable. The veterinarian is about to order a dextrose bolus. The clinic's analyzer uses a glucose oxidase method, and the sample was not separated promptly. Another blood sample is available in a lithium heparin tube collected at the same time and kept at room temperature.
1. What is the most likely explanation for the glucose result, and what should you do before treatment?
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The most likely explanation is in vitro glycolysis: the serum separator tube sat for about 85 minutes before centrifugation, and red blood cells continue to consume glucose after collection. A glucose of 42 mg/dL is inconsistent with a bright, alert cat eating normally and with the rest of the panel. Before giving dextrose, confirm the result with a point-of-care glucometer or by promptly running the heparin sample, recognizing that heparin does not stop glycolysis. Ideally, collect a fresh sample in a fluoride oxalate tube if analysis will be delayed. Do not treat an asymptomatic patient for hypoglycemia based on a single suspect value; treatment can cause hyperglycemia and unnecessary stress.
2. Explain how sample handling could also affect potassium and why you would or would not trust the potassium on this sample.
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Delayed separation can cause pseudohyperkalemia because potassium leaks from red blood cells and platelets into the serum. Hemolysis can also falsely increase potassium. If the potassium is elevated on this sample, it should be repeated on a promptly processed sample before treatment, unless the cat has ECG changes or clinical signs consistent with true hyperkalemia. Conversely, a normal potassium on a delayed sample is more reassuring but still not ideal. The key is to interpret the result in context and to recognize that sample handling affects multiple analytes, not just glucose.
3. Design a simple SOP to prevent this artifact for wellness panels.
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A workable SOP: label every tube with collection time; centrifuge serum separator tubes within 30 minutes of collection; separate serum promptly; if glucose cannot be run within 30 minutes, collect a fluoride oxalate tube for glucose; do not leave tubes on the counter during analyzer downtime; log the time of collection, centrifugation, and analysis; and repeat any result that is clinically inconsistent. Include a rule that critically abnormal glucose values are verified with a second method or fresh sample before treatment. This SOP addresses the whole preanalytical phase, which is where most errors occur.