Interpreting Test Results
Routine urine glucose testing, particularly a morning sample, can be valuable and cost effective. Samples should be measured at the same time each day and the average urine glucose concentration over several days used for assessment. Some glucose in the urine is to be expected in all diabetics. The absence of glucose in the urine suggests insulin overdose.
Spot or sporadic blood glucose testing is generally not useful except when checking for hypoglycemia. Serial glucose measurements or glucose curves are used commonly to corroborate clinical signs during insulin treatment as well as for investigating unstable diabetics. While these can be helpful, their limitations often outweigh the benefits.
Limitations of serial blood glucose measurements
|Numerous drops of capillary blood required|
|Blood samples can be hard to obtain and cause stress for cats|
|Onset, nadir and duration of insulin effects are not fixed and are influenced by many factors|
|Lowest (nadir) and highest blood glucose can easily be missed – especially if sampling is every 2 hours or more|
|Considerable inter-day variation in blood glucose curves|
|Mismatch between clinical signs and results|
In clinic blood glucose curves
Ask the owner how their cat is doing and review their monitoring records. Admit the cat before or just after the morning meal and insulin dose. Weigh the cat and assess body condition score.
To minimize variability either capillary or venous samples should be taken. Capillary blood samples can be taken from the lateral ear margin in cats.
Measure blood glucose at least every 2 hours for as long as possible – ideally for the whole 12-hour dosing interval. If blood glucose falls below 8.3 mmol/l or 150 mg/dl, sample hourly.1
Plot the blood glucose concentrations against time. Look at the pattern and then connect the points. Onset, peak and duration of insulin effect are not fixed – there is considerable between day variation.
- Does the owner think that their cat is happy?
- Is the cat gaining or losing weight?
- Does the cat have worsening clinical signs?
- What is the first blood glucose concentration? Was this before or after insulin?
- Is blood glucose decreasing? When? Is this slow or rapid?
- What is the lowest blood glucose recorded? When does this occur?
- What is the difference between the initial blood glucose and the nadir?
- When does blood glucose return to the baseline (first blood glucose concentration)?
Aim – Blood glucose between 4.5 and 17 mmol/l (81 and 300 mg/dl) for up to 12 hours. The renal threshold for glucose is 14-17 mmol/l (252-300mg/dl).
Glycemic instability – If you document, see clinical signs or suspect hypoglycemia, decrease the insulin dose by at least 50%. You may need to go back to the starting dose of 1 or 2 IU per injection.
Insulin resistance – Is the insulin dose higher than 1.5-2.2 IU/kg? Consider glycemic instability. Investigate underlying causes, e.g., concurrent disease, diabetogenic drugs, compliance with insulin storage, dose and dosage regimen. Read more about Insulin Resistance
Insulin dose too low – Rule out causes of insulin resistance and glycemic instability before carefully and gradually increasing the insulin dose. Read more about Increasing Insulin Dose
Continuous Glucose Monitoring
Continuous glucose monitors (CGMs) are being used increasingly to identify hypoglycemia – which may occur at night at a time apparently unrelated to the insulin injection. These measure interstitial glucose – which lags and is slightly different from blood glucose – but have the advantage that they measure around the clock. The sensor remains in situ under the skin for up to 14 days – in cats up to 5-7 days is a more realistic expectation. CGMs may be more expensive than serial blood glucose measurements. However, the data provided is more valuable and more reliable particularly in cases that need further assessment, like cats that are experiencing hypoglycemia (often occurs at night and apparently unrelated to peak insulin effect), stressed cats and cats with concurrent disease. The newer systems are factory calibrated, easy to use and painless to apply.
Limitations of continuous glucose monitors
|Pressure on sensor can alter readings (e.g. bandaging over sensor, cat lying on sensor)|
|Sensor can dislodge after a few days|
|Expensive – but may be better value than repeated blood glucose curves|
Fructosamine is formed by non-enzymatic, irreversible binding of glucose to albumin.1 In cats, it reflects the mean blood glucose over approximately the last 3 weeks. It can be useful for following up a single unexpectedly high or low blood glucose.1 Fructosamine is used to corroborate clinical signs during insulin treatment but may not be very reliable and should not be used in isolation in diabetics with glycemic instability.1 Fructosamine results >450 µmol/l suggest poor control.2
Limitations of fructosamine
|Considerable variations between analyzers|
|Decreased in hyperthyroidism and hypoproteinemia (but not in azotemia in cats)|
|Serum sample needed – do not use hemolyzed samples|
|May not be available in house|
Glycated hemoglobin (HbA1c) is used routinely in human diabetics. It assesses average blood glucose over 6 weeks and may prove valuable as point of care analyzers validated for veterinary use become available. The value of HbA1c in diabetic cats remains to be established.
References 1. Davison L. In BSAVA Manual of Canine and Feline Clinical Pathology, 3rd edition, Eds E Villiers, J Ristić, 2016, pp. 314-332. 2. Behrend E et al. J Am Anim Hosp Assoc. 2018;54:1–21.