Point-of-care glucose, ketone and urinalysis testing — formative SAQs

SAQ 1 — The neonatal glucose meter and the confirmation rule (15 marks, 15 minutes)

A two-day-old term neonate on parenteral nutrition for poor feeding has a capillary blood glucose meter reading of 1.9 mmol per litre. The meter uses a glucose dehydrogenase strip with a pyrroloquinoline-quinone cofactor. The neonate is asymptomatic. [6]

Question. Describe the chemistry of the capillary glucose meter, explain why this reading may be unreliable in a neonate, state the relationship between the whole-blood and the plasma glucose, and outline the correct next steps.

Model answer

Strip chemistry (3 marks). The capillary blood glucose meter measures the whole-blood glucose by the glucose oxidase or the glucose dehydrogenase electrochemical strip on the capillary sample. The reaction converts the glucose to the gluconic acid and the electrons flow to the electrode to generate the current that the meter reads in five seconds. The glucose oxidase chemistry depends on the oxygen, and the glucose dehydrogenase pyrroloquinoline-quinone variant cross-reacts with the galactose, the maltose, and the xylose. [7]

Sources of error in the neonate (4 marks). The neonate is the highest-risk group for the glucose meter inaccuracy because the high haematocrit biases the reading, the variable oxygen affects the glucose oxidase chemistry, and the galactose, the maltose, or the xylose cross-reacts with the glucose dehydrogenase pyrroloquinoline-quinone strips. The parenteral nutrition and the maltose-containing products are the high-risk exposures, and the galactosaemic neonate is the classic case. The reading can be falsely high or falsely low, and the error is greatest in the neonate and the critically ill child. [6]

Whole-blood versus plasma glucose (3 marks). The capillary whole-blood glucose reads approximately ten to twelve per cent lower than the laboratory plasma glucose because the red cells contain less glucose than the plasma, and the high neonatal haematocrit widens this bias. The device may report a whole-blood or a plasma-calibrated value, and the clinician must know which. The laboratory plasma glucose is the gold standard because the plasma separation removes the haematocrit bias and the laboratory analyser has the tighter precision. [6]

Next steps (5 marks). Confirm the value with a laboratory plasma glucose before acting on the meter reading alone, and treat the symptomatic hypoglycaemia while the result is awaited. Where it is available, the interference-corrected meter such as the Nova StatStrip gives the better accuracy, the better sensitivity, and the reduced test utilisation in the neonatal intensive care. Avoid the milking or the squeezing of the heel that dilutes the sample with the tissue fluid, use the warmed heel and the free-flowing blood, and record the result with the time and the device. The general principle is that the bedside test is the first filter and the laboratory test is the arbiter. [6] [7]