Summer usually means longer days, holidays, travelling and more time outdoors. But higher temperatures also mean that we lose more water, particularly through sweating, and sometimes we simply do not replace it quickly enough.
For most people, mild dehydration is temporary and easily corrected. For someone living with a blood disorder, however, dehydration may deserve a little more attention.
It can change some blood test results, affect blood flow and, in certain haematological conditions, add another stressor to an already vulnerable system.
So, what actually happens to our blood when we become dehydrated?
Dehydration can change your blood test results
Blood is made of cells suspended in a liquid component called plasma. When we lose water and plasma volume decreases, the blood cells have not suddenly multiplied. There is simply less liquid surrounding them.
This is called haemoconcentration.
Experimental studies have shown that significant dehydration, particularly during heat exposure, can reduce plasma volume and influence measurements such as haemoglobin and haematocrit. [1]
This means that dehydration can sometimes make a blood test look different from your usual results.
For example:
- Haemoglobin and haematocrit may appear higher
- Some measured blood components may appear more concentrated
- Electrolytes may change, depending on whether the body has lost mainly water, salt, or both
- With more significant fluid loss, renal function may also be affected
Hydration status influences renal physiology as well as circulating volume, although this becomes more clinically relevant when fluid loss is substantial rather than with every mildly thirsty summer afternoon. [2]
This distinction is important when interpreting laboratory results.
Imagine someone with chronic anaemia whose usual haemoglobin is around 9.5 g/dL. If that person becomes significantly dehydrated, their next haemoglobin measurement may be somewhat higher simply because there is less plasma.
The anaemia has not necessarily improved.
The person may actually feel worse, with fatigue, thirst, headache, dizziness or weakness, while the haemoglobin looks slightly “better” on paper.
Similarly, an unexpectedly high haemoglobin or haematocrit should not automatically be attributed to dehydration. If the abnormality persists, it needs appropriate medical evaluation.
Does dehydration really make the blood “thicker”?
This is where we need to be precise.
Dehydration can produce haemoconcentration, and under some conditions this can alter whole-blood viscosity. Experimental studies using exercise-induced dehydration have demonstrated increased blood viscosity, which improved again during rehydration. [3]
But haemoconcentration, increased viscosity and thrombosis are not the same thing.
It would be too simplistic to say:
dehydration → thick blood → blood clot
Thrombosis is usually the result of several interacting factors, including the underlying disease, age, previous thrombosis, inflammation, immobility, treatments and other cardiovascular risks.
This becomes especially important when we talk about patients with haematological disorders.
Sickle cell disease: why hydration deserves particular attention
Sickle cell disease is probably one of the clearest examples of a haematological condition in which hydration matters.
Sickle red cells already have abnormal rheological properties. Red-cell dehydration increases the intracellular concentration of haemoglobin S, which favours polymerisation and contributes to the rigidity and altered behaviour of sickle cells. Experimental studies have demonstrated the importance of red-cell hydration and extracellular tonicity in these processes. [4,5]
This does not mean that every episode of dehydration will result in a vaso-occlusive crisis, or that every hot day is dangerous.
Weather-related effects in sickle cell disease are actually more complicated than that.
A 2026 paediatric study investigating emergency visits for sickle cell complications found that extreme cold showed the clearest independent association with vaso-occlusive events overall. Higher temperatures and heat-related measures were associated with increased odds of emergency visits for vaso-occlusive events specifically among higher-risk patients, while some heat exposure appeared protective for acute chest syndrome. [6]
So I would not reduce the relationship to “heat causes a crisis.”
However, the biological importance of red-cell hydration remains clear. Avoiding significant dehydration, especially during hot weather, illness or increased physical activity, is a sensible part of caring for someone with sickle cell disease.
Myeloproliferative neoplasms: when haematocrit already matters
Patients with myeloproliferative neoplasms such as polycythaemia vera and essential thrombocythaemia already have a higher risk of arterial and venous thrombosis compared with the general population. [7]
Polycythaemia vera deserves particular attention because haematocrit itself has direct clinical importance.
The CYTO-PV trial demonstrated that patients whose treatment maintained a haematocrit below 45% had significantly fewer cardiovascular deaths and major thrombotic events than patients managed toward a haematocrit of 45% to 50%. [8]
Dehydration does not increase the patient’s actual red-cell mass. What it can do is reduce plasma volume and produce relative haemoconcentration.
For a patient with polycythaemia vera, therefore, avoiding significant dehydration is reasonable.
But again, dehydration should not be presented as the cause of thrombosis. Thrombotic risk in MPNs is considerably more complex and includes age, previous thrombosis, the underlying disease biology, blood counts, cardiovascular risk factors and molecular characteristics.
The message is simpler than the mechanism: if your condition already makes careful control of haematocrit and thrombotic risk important, there is little benefit in adding avoidable dehydration to the picture.
What about CLL and multiple myeloma?
Patients with chronic lymphocytic leukaemia and multiple myeloma also have an increased risk of venous thromboembolism.
A large Danish cohort demonstrated clinically relevant VTE incidence in patients with CLL, while population-based data have shown substantially increased venous and arterial thrombotic risk in multiple myeloma. [9,10]
But the mechanism here is different from the simple idea of “thick blood.”
Cancer itself can alter coagulation. Inflammation, age, mobility, previous thrombosis, comorbidities and treatment all matter. In multiple myeloma, several treatment-related factors can further modify thrombotic risk.
There is not good evidence that ordinary summer dehydration by itself causes thrombosis in CLL or multiple myeloma.
For these patients, dehydration is unlikely to be the main driver of thrombosis. Nevertheless, avoiding significant dehydration remains a sensible precaution in someone who may already have several disease-related, treatment-related or individual thrombotic risk factors.
A few practical precautions during hot weather
There is no single amount of water that is correct for everybody.
Fluid requirements depend on body size, temperature, activity level, diet, medications and underlying medical conditions.
Rather than becoming obsessed with counting glasses, I prefer a few simple principles:
- Drink regularly throughout the day, particularly when temperatures are high.
- Pay additional attention to fluids when you are sweating more than usual or have fever, vomiting or diarrhoea.
- Carry water when travelling or spending long periods outside.
- Avoid prolonged exposure to extreme heat whenever possible.
- During long journeys, remember that immobility itself can contribute to thrombotic risk. Follow the travel advice given by your treating physician.
- Continue prescribed aspirin, anticoagulants and haematological treatments exactly as instructed.
- Do not start aspirin or anticoagulation simply because it is hot outside or because you think you are dehydrated.
- Patients who have been advised to restrict fluids because of kidney disease, heart disease or another condition should not increase their fluid intake substantially without discussing it with their healthcare team.
Patients with sickle cell disease, MPNs, a previous history of thrombosis or active haematological malignancy may also benefit from discussing individual summer and travel precautions with their haematologist.
Most importantly, pay attention when something feels different
Not every headache means dehydration.
Not every swollen leg means thrombosis.
Not every episode of fatigue means that something serious is happening.
But if you live with a blood disorder, new or unusual symptoms should not simply be blamed on the heat.
Contact your healthcare provider if you feel unwell or develop new, persistent or concerning symptoms.
Seek prompt medical assessment particularly for symptoms such as:
- severe or increasing pain
- marked weakness, fainting or persistent dizziness
- very reduced urine output
- persistent vomiting or diarrhoea
- fever
- new pain or swelling affecting one leg
- chest pain
- sudden shortness of breath
- new neurological symptoms
For patients with sickle cell disease, a new or unusually severe pain episode should also prompt contact with the treating team.
Summer should not become something that patients with blood disorders are afraid of.
It simply helps to understand that heat can change our fluid balance, fluid balance can change some of our blood measurements, and in certain haematological diseases these changes may matter more.
Sometimes prevention really can begin with something as simple as remembering the bottle of water next to you.
References
- Jimenez C, Melin B, Koulmann N, Allevard AM, Launay JC, Savourey G. Plasma volume changes during and after acute variations of body hydration level in humans. Eur J Appl Physiol Occup Physiol. 1999;80(1):1-8. doi:10.1007/s004210050550.
- Anastasio P, Cirillo M, Spitali L, Frangiosa A, Pollastro RM, De Santo NG. Level of hydration and renal function in healthy humans. Kidney Int. 2001;60(2):748-756. doi:10.1046/j.1523-1755.2001.060002748.x.
- Chang CQ, Chen YB, Chen ZM, Zhang LT. Effects of a carbohydrate-electrolyte beverage on blood viscosity after dehydration in healthy adults. Chin Med J (Engl). 2010;123(22):3220-3225. doi:10.3760/cma.j.issn.0366-6999.2010.22.009.
- Lew VL, Etzion Z, Bookchin RM. Dehydration response of sickle cells to sickling-induced Ca++ permeabilization. Blood. 2002;99(7):2578-2585. doi:10.1182/blood.V99.7.2578.
- Carden MA, Fay ME, Lu X, et al. Extracellular fluid tonicity impacts sickle red blood cell deformability and adhesion. Blood. 2017;130(24):2654-2663. doi:10.1182/blood-2017-04-780635.
- Gonzalez F, Hall M, Habre R, et al. Weather-Related Exposures and Emergency Department Visits for Pediatric Sickle Cell Complications. Pediatr Emerg Care. 2026;42(5):367-374. doi:10.1097/PEC.0000000000003561.
- Hultcrantz M, Björkholm M, Dickman PW, Landgren O, Derolf ÅR, Kristinsson SY, Andersson TML. Risk for arterial and venous thrombosis in patients with myeloproliferative neoplasms: a population-based cohort study. Ann Intern Med. 2018;168(5):317-325. doi:10.7326/M17-0028.
- Marchioli R, Finazzi G, Specchia G, et al. Cardiovascular events and intensity of treatment in polycythemia vera. N Engl J Med. 2013;368(1):22-33. doi:10.1056/NEJMoa1208500.
- Gade IL, Riddersholm SJ, Christiansen I, et al. Venous thromboembolism in chronic lymphocytic leukemia: a Danish nationwide cohort study. Blood Adv. 2018;2(21):3025-3034. doi:10.1182/bloodadvances.2018023895.
- Kristinsson SY, Pfeiffer RM, Björkholm M, et al. Arterial and venous thrombosis in monoclonal gammopathy of undetermined significance and multiple myeloma: a population-based study. Blood. 2010;115(24):4991-4998. doi:10.1182/blood-2009-11-252072.
Disclaimer
This article is intended for general educational and informational purposes only. It does not replace individual medical assessment, diagnosis or treatment. If you have a haematological condition, follow the recommendations of your treating healthcare team and contact a healthcare professional if you develop new, persistent or concerning symptoms. In an emergency, seek urgent medical care.