What is a normal fasting/plasma (F/P) glucose ratio?

Normal Fasting/Plasma (F/P) Glucose Ratio

The normal fasting/plasma (F/P) glucose ratio is less than 1. 1

Definition and Normal Values

• The F/P glucose ratio refers to the relationship between fasting plasma glucose and postprandial plasma glucose levels 1, 2
• A normal F/P glucose ratio is less than 1, meaning that fasting glucose levels are typically lower than postprandial (after meal) glucose levels 1, 3
• In normal glucose metabolism, fasting plasma glucose levels should be less than 100 mg/dL (5.6 mmol/L) according to American Diabetes Association criteria 1, 3
• Postprandial glucose levels in normal individuals should be less than 140 mg/dL (7.8 mmol/L) two hours after eating 4, 5

Clinical Significance

• The F/P glucose ratio can be used to assess glucose metabolism and identify abnormalities 1, 6
• An inverted ratio (F/P ratio >1) may indicate impaired glucose metabolism or diabetes 3, 1
• In patients with pleuro-peritoneal leaks, the pleural fluid/serum glucose ratio >1 is diagnostic of an exudate, while a ratio <1 indicates a transudate 3
• The postprandial plasma glucose has a closer association with glycosylated hemoglobin (HbA1c) than fasting plasma glucose in diabetic patients 6, 2

Diagnostic Applications

• The relative contribution of postprandial plasma glucose to HbA1c is high (approximately 70%) in patients with good glycemic control (HbA1c <7.3%) 2
• As diabetes control worsens (HbA1c >10.2%), the contribution of fasting plasma glucose increases to about 70% 2
• Monitoring both fasting and postprandial glucose levels provides a more comprehensive assessment of glucose metabolism than either value alone 6, 2

Interpretation in Different Contexts

• For diabetes diagnosis, the American Diabetes Association defines:
  • Normal fasting glucose: <100 mg/dL (5.6 mmol/L) 3, 1
  • Impaired fasting glucose: 100-125 mg/dL (5.6-6.9 mmol/L) 3, 5
  • Diabetes: ≥126 mg/dL (7.0 mmol/L) 3
• The World Health Organization previously used slightly different cutoffs, defining normal fasting glucose as <110 mg/dL (6.1 mmol/L) 1, 7

Clinical Implications

• Even within the normal fasting glucose range (<100 mg/dL), higher values (87-99 mg/dL) are associated with increased risk of developing type 2 diabetes 8
• The F/P glucose ratio should be evaluated in conjunction with absolute glucose values and other risk factors for comprehensive assessment 1, 8
• For accurate measurement, fasting plasma glucose should be obtained after at least 8 hours of no caloric intake 3, 9

Common Pitfalls and Caveats

• The F/P glucose ratio may be affected by medications, stress, illness, and other factors that influence glucose metabolism 1, 9
• In conditions with abnormal red cell turnover (e.g., hemolytic anemia, iron deficiency), glucose criteria should be used exclusively for diabetes diagnosis rather than HbA1c 3, 9
• Laboratory methods and timing of measurements can significantly impact the interpretation of the F/P glucose ratio 1, 9