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]

SAQ 2 — The blood ketone meter, the ISPAD thresholds, and the urine dipstick in the febrile infant (12 marks, 12 minutes)

A six-year-old with type one diabetes arrives vomiting and breathless with a glucose meter reading of 22 mmol per litre. The blood beta-hydroxybutyrate is 5.0 mmol per litre and the venous pH is 7.12. In the next bay, a febrile ten-month-old has a urine dipstick positive for the leukocyte esterase and negative for the nitrite. [1] [2]

Question. State the ISPAD 2022 diabetic ketoacidosis thresholds and the severity grade, explain why the blood beta-hydroxybutyrate is preferred over the urine acetoacetate, and outline the correct approach to the febrile infant with the positive dipstick.

Model answer

ISPAD thresholds and severity (3 marks). The ISPAD 2022 guideline defines the diabetic ketoacidosis by the glucose greater than 11 mmol per litre, the venous pH under 7.3, the bicarbonate under 15, and the beta-hydroxybutyrate greater than 3 mmol per litre or the moderate-to-large ketonuria. The severity is graded by the pH: the mild at 7.2 to 7.3, the moderate at 7.1 to 7.2, and the severe under 7.1. This child, with the pH of 7.12, is the moderate diabetic ketoacidosis. The beta-hydroxybutyrate action bands are the under 0.6 as the normal, the 0.6 to 1.5 as the mild ketosis, and the over 3.0 as the level associated with the ketoacidosis. [1]

Blood ketone over the urine ketone (4 marks). The beta-hydroxybutyrate is the predominant ketone in the diabetic ketoacidosis, produced at a three-to-one to ten-to-one ratio over the acetoacetate, and the blood ketone meter measures it directly through the beta-hydroxybutyrate dehydrogenase reaction. It reflects the current metabolic state and falls with the insulin therapy to track the resolution. The urine pad measures the acetoacetate, lags behind the blood value because it is filtered and concentrated in the bladder over time, and can paradoxically rise during the recovery as the beta-hydroxybutyrate converts to the acetoacetate. This lag is the source of the false reassurance or the false alarm, and the blood ketone is therefore the preferred marker for the diagnosis and the monitoring. [1] [8]

The febrile infant with the positive dipstick (5 marks). The urine dipstick is a screen and the culture confirms the infection. The leukocyte esterase has a sensitivity of approximately 67 to 94 per cent and detects the esterases from the lysed neutrophils. The nitrite has a high specificity of approximately 98 per cent but a low sensitivity of approximately 50 per cent, because the Griess reaction needs the bladder dwell time for the nitrate conversion and the young infant voids frequently. The febrile non-toilet-trained child therefore obtains the culture from the catheterisation or the suprapubic aspiration before any antibiotic, because the adhesive bag specimen is unsuitable for the culture. The antibiotic is started on the clinical probability while the culture grows, and the both-negative dipstick in a well, low-risk child carries the high negative predictive value that supports the observation. The general principle is that the dipstick screens and the culture confirms. [2] [4] [10]