The interpretations below are provided by Donna Castellone, MS, MT (ASCP) SH for Aniara Diagnostica.

What does a prolonged aPTT mean? In a perfect coagulation world- the short answer is it means the patient is on heparin, has a factor deficiency, or an inhibitor (to specific factor, or a lupus anticoagulant). What does a laboratory do? Many will perform a mixing study, which in itself can be a problem- since what constitutes a correction can be different based on what the laboratory uses to interpretate the mix. For this purpose, let us just say the mix corrected or not. If the mix doesn't correct we are pointed in the direction of an inhibitor (lupus, heparin, specific factor, DOACs etc) and if it does correct we investigate a factor deficiency. However, it is not always so simple and these investigations can be costly and time consuming. So what do you do when you have a prolonged aPTT, you do a mix it corrects? You spend a lot of time, looking at all the factors, and come up with all normal results- there could be another path to explore and that is C-Reactive Protein (CRP).



The NIH defines CRP as a protein made by your liver that is sent into your bloodstream in response to inflammation. It is a member of the pentraxin family of protein and responds to cytokines. CRP is also released by muscle cells, macrophages, endothelial cells, lymphocytes and adipocytes. It also plays a role in inflammatory processes by altering pathways involved in apoptosis, phagocytosis, nitric oxide release and production of cytokines.(1)

CRP can elevate up to 1000-fold at the site of infection or inflammation. These causes may be due to bacterial infections (either mild or severe), autoimmune disease, diabetes, smoking, obesity, pregnancy, stress(1) and most recently added to the list COVID-19.

One of the main pathological changes of COVID-19 are lung and immune system damage. Early detection and monitoring of key indicators is important in both diagnosis and treatment strategies as well as the assessment of the severity of the patients' condition. CRP levels correlated with the level of inflammation and can be used in for the early diagnosis of pneumonia with higher levels seen in patients with severe pulmonary infectious diseases. Levels correlate with lung lesions, meaning that in the early stages of COVID-19 it can predict disease severity.(2)



One of the most frequent reason an outpatient is referred for a hemostasis consult is due to a prolonged aPTT. A study that looked at 187 patients found that the most frequent cause was found to be antiphospholipid antibodies (22.6%), contact pathway factor deficiencies (17.4%), other coagulation factor deficiencies (11.6%) and vitamin K deficiency/liver disease. In up to 22.1% of patients there was no identifiable cause found for a prolonged aPTT. In these patients the aPTT while prolonged was significantly less that when a specific cause was found for a prolonged result. Cost for testing ranged from $191.60 to $1055.60.(3)

The clinical impression of these prolonged aPTTs without any cause, seemed to be due to an inflammatory response. CRP is an acute phase reactant and was also elevated in these patients. CRP has an affinity for phospholipids, these are used as a catalytic surface in aPTT based assays and may interfere with the aPTT assay.(4)

It is well known that the aPTT is a heterogeneous assay in that different reagents are made up with different activators and different types of phospholipids which can cause issues during testing. This is seen in the different performance of reagents in proficiency testing.

A study looked at the influence of CRP on the aPTT using 59 patients from the ED who had no bleeding history. Levels of CRP were measures and the aPTT was measured using STA Cephascreen kaolin based assay and a silica based STA aPTT. Both reagents produced similar reference ranges of <32 seconds. The kaolin based aPTT was not impacted by the CRP level, but the silica aPTT was prolonged up to 38 seconds. When normal plasma were spiked with CRP (200mg/L) in vitro, both assays were prolonged suggesting CRP prolongs aPTT assays.(4) Samples were also tested with Actin FS and HemosIL SynthASil in the same concentration of CRP and both resulted in prolonged aPTTs in a dose dependent manner. Using a kaolin based aPTT assay may then be useful in testing these patients.(4)

A study looked at 1859 patients using APTT-SP Hemosil while also measuring hs-CRP, while being classified based on cardiovascular assessment at low risk (< 1.0mg/L) Medium risk (1-3 mg/L) and high risk (>3.0). The aPTT ratio was considered normal at 1.19. Results showed that 18.61% had high risk CRP and prolonged APTT ratio; 57.27% had high risk CRP with normal APTT ratio, 1.24% medium risk CRP and prolonged APTT ratio; 10.99% CRP medium risk and normal APTT ratio; 0.73% CRP low risk and prolonged APTT ratio and 11.16% CRP low risk and normal APTT ratio. There was no correlation between the levels of CRP with abnormal APTT ratio. This study did not show any prolongation based on CRP level.(5)

Another study looked at pooled plasma aPTT with four different reagents with different concentrations of CRP. Baseline aPTTs were 34.5 sec using silica, 30.7 sec activator kaolin, ACL Top 37.7sec and CS-20001 30.6 seconds. The aPTT was prolonged by 24.6 second when the CRP was 300 mg/L and the activator was silica.(6) It appears that the different activator of coagulating factor 12 (F12) has different activating way, which may be result in activable F12 binding different sites of phospholipid.(6)

On the flip side of the coin, using the aPTT to monitor UFH therapy and the influence of CRP can cause a discrepancy. CRP was looked at in ECMO patients along with the aPTT and anti-Xa assays to measure UFH to see the impact of the CRP. Elevated CRP levels were also accompanied by an increase in both fibrinogen and FVIII activity. As a result, the concomitant FVIII increase prevented the increase in the aPTT.(7)



It is important to be aware of the possible interference of CRP on a prolonged aPTT in particular if you are using a silica-based reagent. This should be considered prior to conducting an expensive coagulation work-up.



  1. Levy, J., Treating and Preventing Elevated C-Reactive Protein, September 25, 2019,
  2. Wang, L., C-reactive protein levels in the early stage of COVID-19, Médecine et Maladies Infectieuses, Volume 50, Issue 4, June 2020, Pages 332-334.
  3. Ayla Cristina Nóbrega Barbosa 1, Silmara Aparecida Lima Montalvão 2, Kevan Guilherme Nóbrega Barbosa 3, Marina Pereira Colella 2, Joyce Maria Annichino-Bizzacchi 1 2, Margareth Castro Ozelo 1 2, Erich Vinicius De Paula 1 2. Prolonged APTT of unknown etiology: A systematic evaluation of causes and laboratory resource use in an outpatient hemostasis academic unit, Res Pract Thromb Haemost, 2019 Sep 8;3(4):749-757.
  4. André P. van Rossum , L. Tom Vlasveld , Laura J. M. van den Hoven , Carla W. M. de Wit Ad Castel, False prolongation of the activated partial thromboplastin time (aPTT) in inflammatory patients: interference of C-reactive protein, British Journal of Hematology, January 2012.
  5. Farana Matos M, Finazzo C, Taniguchi E. Interference of C – Reactive Protein Level in Activated Partial Thromboplastin Time [abstract]. Res Pract Thromb Haemost. 2020; 4 (Suppl 1). Accessed February 20, 2021.
  6. Jie Liu 1, Fanfan Li 1, Kuangyi Shu 1, Tao Chen 1, Xiaoou Wang 1, Yaoqi Xie 1, Shanshan Li 1, Zhaohua Zhang 1, Susu Jin 1, Minghua Jiang 1. The analysis of false prolongation of the activated partial thromboplastin time (activator: silica): Interference of C-reactive protein, J Clin Lab Anal., 2018 Oct;32(8):e22571.
  7. Vadim Kostousov, Sridevi Devaraj, Karen Bruzdoski, Lisa Hensch, Shiu‐Ki Hui, Jun Teruya, C-reactive protein‐induced activated partial thromboplastin time prolongation in heparinized samples is attenuated by elevated factor VIII, 19 August 2020.