The unseen haemorrhage: why occult blood loss remains the blind spot of perioperative care

Mechanism of occult blood loss

OBL encompasses several mechanisms. First, and most obviously, there is ongoing, unrecognised bleeding following the end of surgery, as in the case presented by Nazir and Pradeep [1]. Although obvious in this case, this can be particularly difficult to identify, particularly in laparoscopic or robotic surgical procedures [5] or following spinal or pelvic surgery in which retroperitoneal haemorrhage can occur [6]. Secondly, blood sequestration into tissue compartments, particularly in procedures involving extensive dissection such as total joint arthroplasty and major oncological surgery, occurs. In these settings, postoperative ecchymosis and tissue oedema reflect ongoing extravascular accumulation rather than overt haemorrhage. Haemolysis and postoperative haemodilution can obscure the true red-cell deficit, with aggressive crystalloid administration during surgery transiently maintaining blood pressure while diluting haemoglobin concentration, masking the real magnitude of erythrocyte loss. Finally, ongoing low-volume bleeding from cancellous bone surfaces or microvascular beds may persist after wound closure, contributing incrementally to anaemia without triggering obvious hemodynamic changes [7].

Orthopaedic surgery provides perhaps the clearest illustration of OBL. In total hip and knee arthroplasty, hidden blood loss has been reported to account for 30–50% of the total perioperative loss [47]. It therefore remains ‘unmeasured’ and can be easily dismissed if not actively considered in the postoperative period. Any surgery with a large surgical exposure, and therefore a significant surgical bed, should be considered at high risk for OBL.

Cardiac surgery presents a different, but equally relevant, context. Cardiopulmonary bypass induces haemodilution, platelet dysfunction and inflammatory activation, each of which can amplify OBL. Chest tube output may underestimate mediastinal bleeding, particularly when clot retention occurs. Moreover, postoperative haemoglobin trends frequently reflect both dilutional and consumptive components, complicating any interpretation [8]. Without vigilant monitoring and a judicious laboratory reassessment, clinicians may underestimate the cumulative red-cell deficit.

Why it is missed

Failure to recognise OBL is not due to a lack of knowledge, but cognitive bias and system design. There are several reasons why OBL can be missed.

Firstly, haemoglobin is a lagging indicator, if fluid resuscitation is not adequate it can remain deceptively stable for hours. The limitations of relying solely on haemoglobin to recognise blood loss was recognised as long ago as 1983 [9], but it is still commonly used in practice today.

Secondly, the surgical technique can be falsely reassuring. Minimally invasive techniques reduce the surgical insult, improve recovery times and can help to reduce complication rates. However, vascular injury can still occur and be more difficult to identify both intra-operatively and postoperatively [5]. The use of topical haemostatic agents at the time of surgery is now commonplace, but can in itself lead to coagulopathies, exacerbating the problem these agents are designed to address [10]. No topical agent is a substitute for good surgical technique and careful haemostasis.

Furthermore, fragmented postoperative reviews and assumptions about postoperative physiology can all contribute to missed warning signs and failure to rescue.

The clinical consequences of occult blood loss

As discussed in the case presented by Bar Yosef [2], OBL can result in metabolic acidosis, lactataemia and the need for blood product transfusion for several days following surgery. The clinical consequences of OBL extend beyond allogeneic transfusion. Postoperative anaemia has been independently associated with delayed mobilisation, prolonged hospital stay, impaired wound healing and an increased risk of cardiovascular events [11]. In high-risk populations, particularly older adults with limited cardiopulmonary reserve, even moderate reductions in haemoglobin may precipitate myocardial ischaemia or cognitive dysfunction. Importantly, these outcomes may arise despite ‘acceptable’ recorded intraoperative blood loss, underscoring the inadequacy of traditional measurement methods.

Recognition of occult blood loss

The challenge, therefore, lies in improving recognition and maintaining a high index of suspicion when there is clinical evidence of ongoing loss, with or without a drop in Hb. Haemoglobin balance methods that calculate total blood loss, based on changes in Hb and estimated blood volume, have gained traction in research settings [4]. Although imperfect, these models capture hidden losses better than visual estimation alone [12], are simple to use and can aid in accounting for total blood loss both intra-operatively or up to 72 hours postoperatively, and take account of any transfusions given [4713]. Additionally, point-of-care viscoelastic testing offers an insight into coagulopathy that may drive ongoing microvascular bleeding, analysis of which will enable earlier intervention.

Prevention of occult blood loss

Preventive strategies are equally critical. The widespread adoption of antifibrinolytic agents, such as tranexamic acid, has substantially reduced both visible blood loss and OBL across surgical disciplines [14]. The administration of tranexamic acid is now recommended for any patient when there is a risk of bleeding where there is breach of the skin or mucous membranes [15]. Although its value intraoperatively is clear, with very low associated risks, the value of repeated doses or infusions of tranexamic acid following major surgery is unclear. This requires careful multidisciplinary team consideration, especially if high thromboembolic risk exists. Meticulous haemostasis, normothermia maintenance, and restrictive transfusion thresholds further mitigate downstream consequences. Enhanced recovery protocols that limit excessive crystalloid administration may reduce dilution effects, providing a clearer picture of true blood loss, while supporting physiologic stability. The maintenance of a high index of suspicion in at-risk patients, together with enhanced monitoring protocols in enhanced-care or high-dependency settings, can enable the detection of deteriorations early with the provision of appropriate interventions.

Looking in the blind spot

Despite the above, important gaps remain. There is no universally accepted definition of OBL, nor standardised methodology for its measurement. Variability in reporting complicates cross-study comparisons and limits the generalisability of findings. Moreover, while the association between postoperative anaemia and adverse outcomes is well established, causality remains incompletely defined. Whether more aggressive prevention of hidden blood loss translates directly into improved long-term outcomes warrants further prospective investigation.

In the interim, perioperative clinicians need a shift in mindset and practice. OBL should be considered an expected component of major surgery rather than an anomalous occurrence. Anticipatory planning, including preoperative anaemia optimisation, and multidisciplinary blood management programmes offer a proactive framework. A high index of suspicion must be maintained, persistent tachycardia, unexplained pain or subtle haemodynamic instability should prompt immediate review and consideration of OBL or ongoing blood loss. The case report by Bar Yosef [2] focused on active retroperitoneal haemorrhage, but perhaps a more probable mechanism was OBL due to microvascular bleeding, dilution of Hb and blood sequestration following major pelvic oncological surgery. Key to the recognition of OBL is the provision of an appropriate level of care and postoperative monitoring. Early identification and treatment will help to avoid complications and lead to safer surgery with improved outcomes. The consideration of a higher Hb threshold in the first 48–72 hours following orthopaedic, major oncological or other high-risk surgery should be included. A transfusion threshold of 70 g/L for all post-surgical patients may be too low and pick up issues too late for effective prevention and management.

Ultimately, visible blood loss tells only part of the story. OBL is not a new concept but remains a neglected one. As perioperative physicians, we pride ourselves on precision, vigilance and evidence-based practice, yet we continue to underestimate perioperative haemorrhage. Hidden losses, although less dramatic, may exert equally profound physiological effects and contribute to postoperative complications. Recognising their presence requires careful vigilance, systematic measurement and mitigation strategies. As surgical complexity increases and patient populations age, the stakes grow correspondingly higher. OBL is not merely a number; it is a clinically consequential variable that deserves a central place in perioperative risk assessment and management.