Method details, agonists, and where each method breaks

Light-transmission aggregometry (LTA) uses citrated platelet-rich plasma (PRP), stirred while a declared agonist is added, and measures increased light transmission as platelets aggregate and the suspension clears. A trace can show shape change, a lag phase, slope, maximum aggregation, and primary and secondary waves. Agonist selection matters: different agonists probe different activation pathways, so a panel of agonists, not one agonist, is what generates an interpretable pattern. LTA is labor-intensive and sensitive to PRP preparation, agonist concentration, platelet count, hemolysis, and lipemia, which is why it stays a reference method performed in laboratories with the staffing and volume to maintain it well.

Whole-blood impedance aggregometry uses citrated whole blood without a PRP preparation step. Activated platelets adhere to paired electrodes and recruit additional platelets, and the signal is a rise in electrical impedance, reported in ohms or as an instrument-derived area under the curve. Impedance aggregometry preserves red cells and white cells in the test system, which LTA does not, but it has its own platelet-count and hematocrit limitations, and it does not display the shape-change or reversibility information an LTA trace can show. An impedance interval or cutoff is never transferred from an LTA procedure or vice versa; the two outputs are not the same unit or the same signal.

PFA collagen/epinephrine and collagen/ADP cartridges measure closure time, not an aggregation percentage, in citrated whole blood driven through a collagen-coated aperture under high shear. A prolonged closure time on either cartridge may reflect a low platelet count, a low hematocrit, reduced VWF, a medication effect, or intrinsic platelet dysfunction, and the cartridge pair does not distinguish among these by itself. On the declared PFA cartridge pair, isolated collagen/epinephrine prolongation with a normal collagen/ADP result is a pattern compatible with aspirin-like effect, not proof of it. Both closure times prolonged is a nonspecific pattern that needs correlation with the complete blood count (CBC), medication history, VWF testing, and additional platelet studies as indicated. A normal PFA result is not taught as excluding every mild platelet disorder, because the method has a defined sensitivity and scope.

Lumiaggregometry pairs an aggregation measurement with a luciferin-luciferase measurement of adenosine triphosphate (ATP) released from platelet dense granules. Reduced ATP release supports a secretion abnormality only after method-specific confirmation, because lumiaggregometry carries substantial analytic and biologic variability. Flow cytometry measures platelet receptor expression or activation markers, such as P-selectin or activated glycoprotein IIb/IIIa, using fluorescent antibodies on citrated whole blood or a validated platelet preparation, and it can complement aggregation studies rather than replace them. Flow cytometry's activation thresholds, agonists, fixation method, and expression format are all method-specific and require local validation; there is no universal flow cytometry cutoff to teach here.

Every method above depends on getting a specimen the method can actually use. A controlled venipuncture, a correctly filled buffered-citrate tube, gentle mixing, room-temperature handling, and method-specific timing are all required before agonist or aperture testing begins. A hemolyzed specimen, interrupted flow or venous collapse during collection, or visible microthrombi can invalidate or interfere with any of these methods, and medication history, including aspirin and other platelet-active drugs, must be collected as interpretive data before assigning an acquired or inherited pattern to any result.

A limitation belongs to the method that produced the result, so name the method before you explain what a discrepant or borderline number might mean.