What does a platelet reactivity unit (PRU) value of 90 indicate in a patient on antiplatelet therapy, specifically clopidogrel?

What is 90 PRU?

A PRU (Platelet Reactivity Unit) value of 90 indicates excellent platelet inhibition in a patient on clopidogrel, representing a therapeutic response well below the threshold for high on-treatment platelet reactivity (HTPR), and is associated with minimal thrombotic risk but warrants monitoring for potential bleeding complications.

Understanding PRU Values and Clinical Thresholds

PRU is measured using the VerifyNow P2Y12 assay, a point-of-care test that quantifies residual platelet reactivity in patients taking P2Y12 inhibitors like clopidogrel 1. The measurement reflects how much platelet function remains despite antiplatelet therapy.

Established Cut-off Values

High on-treatment platelet reactivity (HTPR) thresholds:

• PRU ≥235 is associated with significantly increased risk of cardiovascular death, stent thrombosis, and major adverse cardiac events after drug-eluting stent implantation 1
• PRU ≥194 defines clopidogrel hyporesponsiveness in some studies 2
• PRU >240 identifies hypo-responders at 24 hours post-loading dose 3, 4

Hyper-response (excessive inhibition) thresholds:

• PRU <60-70 defines platelet hyper-responsiveness and is associated with increased hemorrhagic complications 3

Clinical Interpretation of PRU 90

A value of 90 PRU places the patient in the optimal therapeutic range - well below the HTPR threshold but above the hyper-response bleeding risk zone 3, 1. This indicates:

• Thrombotic risk: Minimal. The patient has adequate platelet inhibition, with PRU far below the 194-240 range associated with increased thrombotic events 2, 3, 1
• Bleeding risk: Low to moderate. While not in the highest-risk category (<60 PRU), values in this range still represent substantial platelet inhibition 3

Clinical Context and Implications

Post-PCI with Drug-Eluting Stents

In patients after percutaneous coronary intervention, PRU 90 represents excellent protection against stent thrombosis. Studies demonstrate that PRU >235 carries a 6.5% rate of combined cardiovascular death, MI, or stent thrombosis versus 1.0% in patients with lower PRU values 1. Your patient with PRU 90 falls into the lower-risk category.

Acute Coronary Syndrome Patients

Mean post-treatment PRU values in stable patients typically range 184-190 1, 4. A PRU of 90 indicates more robust platelet inhibition than average, which may be beneficial in high-risk scenarios but requires bleeding vigilance 3.

Cerebrovascular Disease Considerations

In acute ischemic stroke patients, PRU values tend to be higher (mean 270 at 24 hours post-loading) compared to other cerebrovascular diseases (mean 190) 4. A PRU of 90 in this population would be exceptionally low and uncommon, potentially indicating either excellent clopidogrel metabolism or need for bleeding risk assessment 4.

Important Clinical Pitfalls

Do not assume PRU values alone predict clinical outcomes in all contexts. While PRU hyporesponsiveness (>194-240) clearly associates with thrombotic events, and hyper-response (<60-70) with hemorrhagic events 3, 1, the predictive value has limitations:

• In cerebrovascular procedures, PRU stratification did not significantly differentiate thrombotic or hemorrhagic complication rates in some studies 2
• Baseline PRU values, not CYP2C19 genotype, better predict clopidogrel response 4
• Clinical factors (diabetes, obesity, fusiform aneurysms, posterior circulation procedures, multiple stents) may outweigh PRU values in predicting thrombotic complications 2

Avoid premature clopidogrel discontinuation. Even with excellent platelet inhibition (PRU 90), stopping clopidogrel within 12 months post-ACS or before minimum stent duration significantly increases stent thrombosis risk regardless of PRU values 5, 6.

Monitor for bleeding symptoms, particularly in the first 3 months of therapy, as PRU 90 represents substantial platelet inhibition 7. Consider gastrointestinal prophylaxis if additional bleeding risk factors exist 8.

Management Algorithm for PRU 90

1. Confirm indication for clopidogrel: Recent ACS, PCI with stenting, or cerebrovascular disease 5, 6
2. Continue current clopidogrel 75 mg daily - no dose adjustment needed 9, 1
3. Assess bleeding risk factors: Age >75, prior bleeding, concurrent anticoagulation, renal dysfunction 8, 5
4. If on triple therapy (aspirin + clopidogrel + anticoagulant): Consider stopping aspirin early while continuing clopidogrel plus anticoagulation to reduce bleeding risk 5
5. Recheck PRU only if clinical indication changes (recurrent ischemic events, significant bleeding, medication changes affecting CYP2C19) 6, 4