What blood tests can be performed using capillary blood samples in patients across different demographics, including those with diabetes or cardiovascular diseases?

Blood Tests Performed Using Capillary Blood Samples

Capillary blood samples obtained by finger-stick can be used for glucose monitoring in established diabetes patients, HbA1c measurement, lipid profiles, and complete blood counts in stable outpatients, but should never be used for diagnosing diabetes and must be avoided in critically ill patients, those in shock, on vasopressors, or with severe peripheral edema. 1, 2

Glucose Monitoring

Appropriate Use

• Capillary glucose testing is valuable for diabetes management in patients with established diabetes, providing real-time feedback for insulin dose adjustments and treatment decisions 3, 1
• Point-of-care glucose monitoring should be performed before meals in hospitalized patients who are eating, or every 4-6 hours in those not eating 1
• Patients on insulin therapy should monitor capillary glucose levels actively, with those on basal insulin focusing on fasting levels and those on premixed insulin monitoring both fasting and pre-dinner levels 1

Critical Limitations and Contraindications

• Portable glucose meters using capillary blood should never be used to diagnose diabetes because diagnostic criteria are based on venous plasma glucose from accredited laboratories 1, 3
• Capillary glucose measurements become unreliable in shock, severe hypotension, poor peripheral perfusion, severe peripheral edema, or mottled skin 1, 3, 2
• In critically ill patients on vasopressor therapy, arterial or venous whole blood sampling must be used instead of finger-stick capillary testing 1
• Diabetic ketoacidosis and hyperglycemic hyperosmolar states make capillary measurements unreliable due to microcirculatory obstruction 3

Technical Accuracy Considerations

• Capillary whole blood values differ from venous plasma values, particularly in the post-load state more than fasting state 1
• A plasma-calibrated meter reports readings close to laboratory results when fasting, but slightly higher than venous glucose postprandially 1
• Glucose meters show mean absolute relative differences ranging from 5.6% to 20.8%, with less than half meeting accuracy standards 3
• In hospital settings, 45.6% of split samples differed by more than 10% between meters and laboratory analyzers, with 14% differing by more than 20% 3

HbA1c Testing

Clinical Application

• Capillary HbA1c measurements are an accurate alternative to venous measurements when analyzed using proper laboratory methods 4
• In a large study of 773 paired samples, 97.7% of capillary samples were within 5% of their respective venous measurement, with R² of 0.95 4
• The DCA 2000 instrument can evaluate HbA1c using capillary blood with excellent precision, though values may be slightly lower than HPLC systems 5

Important Caveats

• Chronic renal failure significantly interferes with capillary HbA1c accuracy, showing very low correlation (r = 0.203) between capillary and standard methods 5
• Hyperlipemia shows acceptable but reduced correlation (r = 0.876) compared to normal samples 5

Lipid Profile Testing

Performance Characteristics

• Point-of-care devices can measure total cholesterol, triglycerides, and HDL-cholesterol from capillary blood with acceptable accuracy 6
• Correlation coefficients for capillary blood lipid measurements are: TC (R=0.965), TG (R=0.969), and HDL-C (R=0.943) 6
• Total errors for lipid profiles using capillary blood are acceptable: TC (6.6%), TG (9.3%), and HDL-C (11.6%) 6

Complete Blood Count Parameters

Acceptable Parameters

• Most hematologic parameters show no significant bias between capillary and venous samples when analyzed on automated clinical analyzers 7
• Regression analysis demonstrates acceptable agreement for the majority of studied hematologic parameters 7

Parameters Requiring Caution

• Mean corpuscular volume (MCV) shows significant bias between capillary and venous samples and should be interpreted cautiously 7
• If samples cannot be analyzed within 24 hours, the following parameters become unreliable: hematocrit, MCV, mean corpuscular hemoglobin concentration, platelet distribution width, mean platelet volume, and basophils 7

Critical Contraindications

• Never use capillary sampling in critically ill patients requiring invasive vascular monitoring, as it produces clinically significant errors 2
• In patients with shock, hypotension, or receiving vasopressor therapy, capillary sampling creates large and unpredictable errors 2
• Severe peripheral edema or mottled skin makes capillary samples unreliable and dramatically different from true systemic values 2

Additional Biochemistry Parameters

General Performance

• 22 common biochemistry parameters can be measured from capillary blood with no significant bias compared to venous samples 7
• Mean hemolysis index is higher in capillary samples compared to venous samples, which may affect certain analytes 7

Stability Concerns

• Ferritin and vitamin D show percentage deviations exceeding analytical performance specifications after 24 hours at room temperature 7
• Samples should be analyzed promptly or proper storage conditions maintained 7

Practical Considerations

Patient Preference

• Finger pricking is perceived as less painful than venipuncture in participants who undergo more than one blood test per year 7
• Capillary blood collection is minimally invasive and suitable for resource-limited settings 8

Quality Control Requirements

• Rigorous quality control procedures must be used when health care providers perform capillary blood testing 1
• Participation in voluntary proficiency testing programs (such as College of American Pathologists) is recommended 1
• Any glucose result that does not correlate with the patient's clinical status should be confirmed by measuring a serum sample in the clinical laboratory 1