Table 4.

Overview of selected testing in hemostasis.

Test category	Test	Blood volume required	Venue performed	Advantages and limitations
Platelet evaluation	Platelet count (58–62)	• Clinical laboratory: 1 mL whole blood • Sample test volume: ≤ 250 μL whole blood	Clinical laboratory	Advantages: • Has common clinical use with validated standardized procedures • Is simple to perform either manually or through automated techniques Disadvantages: • Does not provide information about platelet function • Can be time intensive (primarily manual counts) • Overestimates when cellular debris or other cell populations are present • Underestimates in samples with enlarged platelets or platelet clumping • Has limited accuracy, particularly very low platelet counts (<10,000/μL)
	Light transmission aggregometry (57–70)	• Clinical laboratory: 20 mL whole blood • Sample test volume: 3–5 mL platelet-rich plasma or whole blood	Clinical laboratory	Advantages: • Tests specific platelet function responses to a panel of agonist • May be augmented with light scatter techniques to better capture the early phase of aggregation and assess aggregates of different sizes Disadvantages: • Is time and resource intensive • Requires special expertise and training to interpret • Requires large blood volumes, particularly for the pediatric population • May be unreliable if the initial specimen contains platelet aggregates
	Bleeding time (71–74)	NA	Point-of-care	Advantages: Evaluates in vitro platelet function through in vivo test Does not require specialized equipment to perform Disadvantages: Is time and resource intensive Requires an invasive procedure Requires specialized training to perform (manual method) Has variable reproducibility Has unclear ability to predict bleeding risk unless grossly abnormal
	PFA-100/PFA-200 (74–76)	• Clinical laboratory: 3–5 mL whole blood • Sample test volume: 1–3 mL whole blood	Point-of-care	Advantages: • May provide a more standardized approach than other platelet function testing • Measures platelets at high (physiologic) shear rates Disadvantages: • Is time and resource intensive • Provides unclear association of platelet function and bleeding risk
	Cone and plate analyzer (74, 77)	• Clinical laboratory: 3–5 mL whole blood • Sample test volume: 150–250 μL whole blood	Clinical laboratory	Advantages: • Provides platelet function information using a small blood volume • Uses a variety of specific agonists which allows for a variety of applications • Yields results rapidly (within 15 min) • Allows for platelet function to be measured even at low platelet counts Disadvantages: • Is influenced by red cell count • Is manually conducted and may have significant operator variability • Provides unclear association of platelet function and bleeding risk
Clotting assays	aPTT, PT, TT (78–82)	• Clinical laboratory: 3 mL whole blood • Sample test volume: 1 mL of plasma	Clinical laboratory	Advantages: • Has common clinical use that is well-standardized and validated • Are helpful as a screening assessment for bleeding and thrombotic disorders • Are often utilized in diagnostic decision support and therapeutic algorithms • Are easy to perform either manually using the tilt-tube technique or automatedly using high throughput analyzers Disadvantages: • Are non-physiologic tests that oversimplify coagulation pathways • Are affected by in vitro and in vivo factors that do not have any effect on in vivo clot formation • Provides unclear assessment of bleeding risk
	Factor assays (78, 81)	• Clinical laboratory: 3 mL whole blood • Sample test volume: 0.5 mL of plasma	Clinical laboratory	Advantages: • Has common clinical use that is well-standardized and validated • Is useful in the diagnosis of specific protein alterations that may cause or contribute to bleeding and thrombosis disorders Disadvantages: • Requires a large blood volume for pediatric patients, particularly if testing multiple factor levels • Affected by in vitro and in vivo factors that do not have any effect on in vivo clot formation
Global measures	Viscoelastic testing (83–96)	• Clinical laboratory: 3 mL whole blood • Sample test volume: ≤ 400 μL whole blood	Point-of-care	Advantages: • Has rapid turn-around time and may be performed at the bedside • Is more cost-effective than standard measurements of hemostasis • May decrease use of blood products in certain populations Disadvantages: • Requires operator training in interpretation • Requires rapid processing of sample (within 3 min) for some measures • Has poor precision with high coefficient of variance • Is unclear in ability to predict bleeding risk
	Exogenous thrombin potential (97, 98)	• Sample test volume: ≤ 500 μL platelet-rich or platelet-poor plasma	Primarily research	Advantages: • Is reflective of the physiologic state, assessing activation, clot formation, and fibrinolysis • Allows for different antibiotics, proteins, and enzymes to interrogate different aspects of hemostasis Disadvantages: • Does not have fully established and validated reference ranges • Is not readily available for clinical use
	Sonic estimation of elasticity resonance (99–102)	• Sample test volume: 1.5 mL whole blood	Primarily research	Advantages: • Provides a direct estimate of mechanical clot properties Disadvantages: • Does not have fully established and validated reference ranges • Is not readily available for clinical use

aPTT, activated partial thromboplastin time; PT, prothrombin time; TT, thrombin clotting time.