BLOOD SCIENCES Early inflammatory phase
When bacteria or their toxins enter the bloodstream, they activate immune responses, particularly via hepatic, signalling pathways. This leads to increased synthesis of acute-phase proteins, most notably fibrinogen. As fibrinogen is present in plasma but absent in serum, plasma viscosity rises disproportionately compared to serum viscosity, resulting in a distinctive increase in the PV:SV ratio.
Established sepsis As the inflammatory response progresses, both plasma and serum protein concentrations will increase; however, fibrinogen remains a dominant contributor to elevating the plasma viscosity. The PV:SV ratio may remain elevated but becomes more variable due to concurrent changes in immunoglobulins, albumin, and fluid balance.
Late phase / consumptive coagulopathy
In severe sepsis, DIC will develop, characterised by widespread microvascular thrombosis and rapid consumption of cloting factors, including fibrinogen and platelets. As fibrinogen levels decline, plasma viscosity decreases, causing the PV:SV ratio to dramatically fall towards the baseline or lower. This reduction will signal transition to a critical coagulopathic state.
Confirming DIC using PV:SV Predicting DIC in sepsis isn’t just about anticipating a complication. Confirming DIC fundamentally shifts care from reactive to proactive, which will significantly improve sepsis outcomes by: early intervention and treatment reduction in mortality improved monitoring and resource allocation prevention of complications more informed clinical decision-making enhanced organ protection shorter hospital stays and reduced healthcare costs implementation of a precision medicine approach.
PV:SV ratio Phase ~1.06
~1.2 ~1.3–1.4 ~1.2 ~1.1 (or lower) Baseline
Early acute phase
Peak
hyperinflammatory phase
Consumption phase
Late / DIC phase
Declining ratio
Low–normal or reduced ratio
Dynamic changes in PV:SV during sepsis progression.
Discussion It is universally agreed that a rapid diagnosis is essential in cases of suspected sepsis. A low diastolic blood pressure in conjunction with clinical signs and symptoms suggestive of sepsis, should alert the clinician towards a positive sepsis diagnosis. To confirm the diagnosis of sepsis,
NICE recommends the following range of diagnostic and confirmatory investigations: measurement of lactate levels liver function tests full blood count with differential white cell count blood film coagulation screen blood cultures to rule out septicaemia inflammatory markers such as PV or CRP.
To complete these investigations will require significant resources and most importantly time, a precious factor many sepsis patients will not have. Plasma viscosity testing can be
completed within minutes; it should therefore be prioritised in the initial clinical assessment. A positive result supports the early recognition of sepsis and enables the prompt initiation of appropriate management and therapeutic intervention. The 2026 NICE guidelines recommend performing blood cultures in suspected
sepsis.24 Accuracy of the test relies on
proper timing and adequate blood volume. Results to identify septicaemia and its cause may take up to 24 to 48 hours. Plasma viscosity cannot replace blood cultures but can indicate increased immune activity and support prompt initiation of prophylactic treatment. Clinical evidence indicates that plasma
viscosity is a valuable biomarker for the rapid diagnosis of sepsis and other conditions that are often challenging to identify. This was demonstrated during the COVID-19 pandemic, when plasma viscosity proved particularly useful in predicting which patients were likely to require hospitalisation and further intervention.25
Although COVID-19 and
sepsis have different underlying causes, they share several similarities in their immune responses. The relationship between plasma and
serum viscosity directly reflects changes in the plasma proteins, particularly fibrinogen, and provides an indirect insight into the inflammatory status. The use of PV:SV ratio is a quick, reliable tool for fibrinogen quantification and for predicting dynamic changes during sepsis progression and DIC.26
Conclusions Sepsis remains a life-threatening condition with high morbidity and mortality, demanding rapid recognition and decisive diagnostic action. Laboratory investigations are central to this process, providing the clarity needed to initiate timely and effective management. Plasma viscosity testing provides
Plasma viscosity cannot replace blood
cultures but can indicate increased immune activity and support prompt initiation of prophylactic treatment
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reliable, repeatable, and reproducible results that serve as a valuable marker for diagnosing and monitoring the progression of sepsis. Knowing the dynamic changes in circulating plasma proteins, including fibrinogen, accurately reveals the patient’s in vivo inflammatory state and strengthens clinical decision- making.
Interpretation Mechanism Normal ratio
Rising ratio Maximum ratio
Physiological levels of fibrinogen and serum proteins
Fibrinogen increases more than serum proteins (plasma viscosity rises more)
Significant increase in fibrinogen and cloting factors due to peak inflammation
Fibrinogen consumption due to ongoing coagulation
Fibrinogen depletion (eg DIC) reduces plasma viscosity; serum relatively preserved
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