Requirements:
(1) knowledge of the concentration of glucose n the infusing solution
(2) concentration of glucose in the sample collected from the line
(3) volume of sample collected from the line (which is a mixture of true venous blood and contaminating fluid from infusate)
(4) concentration of glucose in a simultaneously collected fingerstick sample
amount of glucose in blood sample in mg =
= (concentration of glucose in sample in mg/dL) * ((volume collected in mL) / 100) =
= ((concentration of glucose in infusion solution per dL) * ((volume of contaminant in mL) / 100)) + ((actual concentration of glucose in blood by fingerstick in mg/dL) * ((volume of blood in mL) / 100))
where:
• An infusion of D5W is 5% glucose, or 5 grams per dL or 5000 mg/dL.
• Division by 100 converts from mL to dL.
Multiplying both sides by 100:
amount of glucose in blood sample in mg =
= (concentration of glucose in sample in mg/dL) * (volume collected in mL) =
= ((concentration of glucose in infusion solution per dL) * (volume of contaminant in mL)) + ((actual concentration of glucose in blood by fingerstick in mg/dL) * (volume of blood in mL))
Also:
volume of sample collected in mL =
= (volume of contaminant in mL) + (volume of blood in mL)
After solving these simultaneous equations:
volume of contaminating solution in mL =
= ((((glucose concentration in sample) / (glucose concentration in fingerstick)) - 1) * (volume of sample in mL)) / (((glucose concentration in infusate) / (glucose concentration in fingerstick)) - 1)
Limitations:
• This method requires a good fingerstick sample to serve as the gold standard. The presence of severe edema or an improperly collected sample can result in a sample inadequate for this purpose.
