What are the considerations for choosing between capillary blood and whole blood collection for laboratory tests in patients of different demographics, such as pediatric, geriatric, and adult populations?

Capillary Blood vs Whole Blood Collection: Clinical Guidance

Primary Recommendation

For diagnostic testing and critical clinical decisions, venous whole blood collection via venipuncture should be the standard approach, with capillary blood sampling reserved for specific situations where venous access is impractical and the patient is hemodynamically stable. 1, 2, 3

Clinical Context and Decision Algorithm

When Venous Whole Blood is Mandatory

Capillary sampling must be avoided in the following situations:

• Critically ill patients requiring invasive vascular monitoring - capillary sampling is inaccurate and leads to clinically significant errors 2
• Patients in shock, hypotension, or receiving vasopressor therapy - capillary samples produce large and unpredictable errors compared to arterial or central venous samples 2
• Severe peripheral edema or mottled skin - capillary samples differ dramatically from true systemic values 2
• Compromised peripheral perfusion of any kind - microcirculatory dysfunction creates unpredictable measurement errors 2
• Diabetic ketoacidosis or hyperglycemic hyperosmolar states - microcirculatory obstruction at sampling sites makes capillary measurements unreliable 3
• Diabetes diagnosis - diagnostic criteria are based on venous plasma glucose from accredited laboratories, not capillary whole blood 3

Sampling Site Hierarchy for Critical Care

The Society of Critical Care Medicine recommends this specific algorithm: 2

1. First choice: Arterial line sampling
2. Second choice: Central venous catheter
3. Third choice: Peripheral venous catheter
4. Last resort only: Capillary sampling (only in stable patients)

When using venous catheters, discard adequate dead space volume and ensure no contamination from glucose-containing or other IV fluids through multilumen catheters 2

Acceptable Uses of Capillary Blood Sampling

HbA1c Testing

Both venipuncture and fingerstick capillary sampling are acceptable for HbA1c measurement when proper collection systems are used 1. Blood tubes should contain anticoagulant as specified by the manufacturer (EDTA unless otherwise specified) 1.

Important considerations:

• Capillary blood collection systems (filter paper, capillary tubes, small vials with stabilizing/lysing reagent) should only be used if studies have established comparability with standard collection methods for the specific assay employed 1
• Whole blood samples are stable for up to 1 week at 4°C 1
• The accuracy of capillary collection methods has been validated in several large research cohorts 1

Point-of-Care INR Monitoring in Pediatrics

Point-of-care INR devices using fingerstick capillary whole blood are acceptable for anticoagulation monitoring in children 1. These devices require minimal blood volume, produce results within 1 minute, and have been shown to improve quality of anticoagulation control and reduce thromboembolic and hemorrhagic events compared to laboratory-based monitoring 1.

Key technical points:

• Differences between laboratory-based and POC-based INR measurements are typically <15%, similar to differences between different laboratories 1
• Results are considered comparable if both INRs are within the targeted range or within 0.5 units 1

Glucose Monitoring (Not Diagnosis)

Capillary glucose monitoring is valuable for diabetes management in established patients to guide therapy adjustments and improve metabolic control 3. However, portable glucose meters should never be used for diabetes diagnosis 3.

Critical limitations:

• A 2017 study of 17 commercially available meters showed mean absolute relative differences ranging from 5.6% to 20.8% 3
• Less than half of 18 commercially available meters met 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

Population-Specific Considerations

Pediatric Populations

For routine diagnostic testing in children, venipuncture remains the standard 1. However, capillary sampling may be more practical for:

• Point-of-care INR monitoring in children on anticoagulation 1
• HbA1c testing when validated collection systems are used 1
• Situations where venous access is difficult and the child is hemodynamically stable 1, 4

Critical requirement: Diagnostic laboratories processing pediatric samples must use age-, analyzer-, and reagent-appropriate reference ranges 1. Age groups should be standardized (neonates, 1 month to 1 year, 1-5 years, 6-10 years, 11-16 years) as these reflect statistically different groupings 1.

Geriatric Populations

Venipuncture should remain the standard in geriatric patients 1. Recent research shows self-collection success rates decline significantly with age - 94.7% for patients under 70 years versus only 25% for patients 80 years and older 5.

Technical Considerations for Glucose Testing

Sample Handling for Fasting Plasma Glucose

Blood for fasting plasma glucose should be drawn after overnight fasting (at least 8 hours) 1. To minimize glycolysis:

• Place the sample tube immediately in an ice-water slurry and separate plasma from cells within 30 minutes, OR
• Use tubes containing rapidly effective glycolysis inhibitors such as citrate buffer 1
• Tubes with only enolase inhibitors (sodium fluoride) should not be relied upon to prevent glycolysis 1

Plasma is recommended over whole blood for glucose diagnosis because glucose concentrations are approximately 11% higher in plasma than whole blood when hematocrit is normal 1.

Common Pitfalls and How to Avoid Them

Blood Draw Device Selection

Use 21-gauge needles for blood draw in adults when possible 1. IV catheters and smaller-bore needles have higher hemolysis rates 1. New straight needle venipuncture is preferred over IV start 1.

Tube Selection and Handling

Use the same brand of tubes consistently throughout studies and limit the number of lots to as few as possible 1. Various tube components (surface coating, stopper materials, gel separators, additives) can interfere with clinical laboratory assays 1.

Standardization Requirements

Record critical information: blood draw time, fasting status, date and time of last meal, hours of sleep the previous night, pre-blood draw exercise activity, and medication use 1. Blood should ideally be drawn at the same time of day for all participants with the same fasting status 1.