OC 26.3 - PLASMIC score to differentiate TTP from other causes of microangiopathic haemolytic anaemia

Background: Microangiopathic haemolytic anaemias comprise a group of serious disorders. Timely distinction of TTP is crucial for urgent plasma exchange to avoid rapid morbidity and mortality. While definitive diagnosis is via ADAMTS-13 testing, turnaround time can be lengthy especially when not locally available. The PLASMIC score was developed to prioritise testing, but its real-world effectiveness requires evaluation.

Aims: To assess diagnostic utility of PLASMIC score retrospectively in patients with thrombotic microangiopathy.

Methods: Consecutive, unselected patients with a new diagnosis of TTP were obtained from the UK TTP registry, and ‘Non-TTP’ (other TMAs) from the TTP registry screening log from 2016 onwards from 3 sites. The PLASMIC score was applied retrospectively in both cohorts. We compared individual PLASMIC score parameters and performed subgroup analyses based on diagnosis and age.

Results: 185 patients with confirmed TTP and 303 ‘Non-TTP’ patients were included in the analysis. The median PLASMIC score was higher in TTP (6) compared with Non-TTP (4) groups (p < 0.0001).

However, 16% of confirmed TTP cases had a score < 5, suggesting the score alone cannot reliably rule out TTP. 45% of non-TTP cases had a score ≥5, potentially leading to unnecessary testing. Platelet count < 30x10^9/L, bilirubin >32umol/L and reticulocytes >2.5% were the only parameters of the 7-point score with statistical significance between TTP and Non-TTP groups.

The PLASMIC score’s sensitivity was 84%, specificity 55%, PPV 53.5% [95% CI 0.47-0.59] and NPV 84.9% [CI 0.79-0.90]. This did not alter when analysing subgroups based on age ≥60 or < 60.

Conclusion(s): Reliance on PLASMIC score < 5 alone to exclude TTP is not sufficient in a significant number of patients. Clinical presentation, particularly MAHA with neurological or cardiac features, should be considered when deciding on rapid plasma exchange and availability of rapid ADAMTS 13 measurement to avoid the potentially devastating consequences of missed TTP diagnosis.