Plasma protein ketoamines
Glucose is able to bind a variety of structures, including proteins, in a non-enzymatic irreversible reaction, a process called glycosylation. When glucose binds in this fashion to amino groups on serum proteins, especially albumin, the glycosylated protein product is called fructosamine. The half-life of fructosamine molecules is directly dependent on the half-life of serum proteins, primarily albumin, which is generally 2-3 weeks in dogs and cats. Therefore, fructosamine is an indicator of the blood glucose concentration over a longer period of time than a single blood glucose measurement (which reflects the glucose concentration at the time of blood collection).
Fructosamine measurement is used for the following in dogs and cats primarily:
- Diagnosis of diabetes mellitus: Fructosamine concentrations above reference intervals indicate a persistent hyperglycemia. This most consistently occurs in diabetes mellitus. On rare occasions, fructosamine can be increased in cats and dogs with chronic (long standing) stress hyperglycemias. Short-term stress hyperglycemias will not elevate blood glucose concentrations for a long enough period of time to elevate serum fructosamine concentrations. Therefore, measurement of fructosamine is useful for differentiating between diabetes mellitus and stress hyperglycemias in small animal patients. Measurement of fructosamine concentrations is useful for small animal practitioners when there are long delays between sample collection and blood glucose assay. Blood glucose concentrations decrease with storage due to consumption by cells (this decrease is accelerated if serum or plasma is not separated from cells after sample collection). Therefore, glucose concentrations may be reduced to within reference intervals in aged samples from diabetic patients. In these cases, fructosamine values are consistently elevated.
- Response to insulin therapy: Fructosamine is also useful for monitoring response to therapy in diabetic patients as persistent changes in glucose will be reflected more rapidly in fructosamine, compared to glycosylated hemoglobin, because serum proteins have a shorter half-life than hemoglobin. Indeed, fructosamine is lower (and often within reference intervals) in well-controlled versus poorly-controlled diabetic small animal patients.
Various methods for detecting fructosamine levels in serum are currently available. The most commonly used method for measuring fructosamine concentration is the nitroblue tetrazolium assay, and is the method used at Cornell University.
Blanked end-point reaction
Procedure for nitroblue tetrazolium assay
In alkaline conditions, ketoamines rearrange to the eneaminol form, which reduces the chemical compound nitroblue tetrazolium, in a one-step reaction, to formazan. A ten minute preincubation period is recommended to circumvent interference effects from fast-reacting, nonspecific reducing substances. The formation of formazan is quantified from an absorbance change measured photometrically, at two different time points, at 540nm and is directly proportional to fructosamine concentration (mmol/L).
Units of measurement
Fructosamine concentration in serum is measured in mmol/L (SI unit).
Reference intervals for fructosamine concentration in some domestic species have been established by the Clinical Pathology Laboratory in the Animal Health Diagnostic Center at Cornell University.
Heparin or EDTA
Plasma or serum
Fructosamine concentrations in serum are quite stable over days to a few weeks at 4°C.
- Lipemia, hemolysis, icterus: Per the product information sheet, lipemia and icterus do not interfere. A hemolytic index >100 may affect results (unknown if increase or decrease).
- Drugs: At therapeutic concentrations, Levodopa results in spuriously high fructosamine levels.
- Diabetes mellitus: Persistent hyperglycemia due to insulin deficiency and/or insulin resistance leads to the glycosylation of serum proteins, which results in fructosamine levels above reference intervals.
- Hypothyroidism: Studies conducted in people revealed that the increase in fructosamine concentration, from hypothyroidism, is caused by a reduction in protein turnover (e.g., increased albumin half-life). This has not been reported in animals.
- Artifact: Fructosamine concentrations may be falsely lower in severe hypoproteinemia; however values appear to be unaffected when serum protein fluctuates within reference intervals.
- Insulinoma: Over-secretion of insulin results in persistent hypoglycemia, decreasing fructosamine levels. However, fructosamine is not recommended as a test for this insulin-producing tumor.
- Hyperthyroidism: In cats with hyperthyroidism, fructosamine levels were significantly lower than healthy cats. This could be due to increased catabolism of proteins.
- Clinical Pathology Laboratory in the Animal Health Diagnostic Laboratory at Cornell University website: Information on testing for fructosamine.