Raised haematocrit (Hct) may reflect genuine raised red cell mass (RCM) or may be spurious due to sample factors (e.g. prolonged sample storage) or patient factors (e.g. dehydration).  Raised RCM can be attributed to a variety of causes due to primary (eg, polycythaemia vera [PV - previously PRV], myeloproliferative neoplasm [MPN]) or secondary factors.  ‘Idiopathic erythrocytosis’ describes the small number of cases where no primary or secondary cause is identified.

True polycythaemia is important clinically due to the thrombotic risk associated with a high haematocrit.

Secondary causes include:

• Alcohol excess
• Hypoxia, respiratory disease including sleep apnoea, smoking
• Renal disease
• Cyanotic heart disease
• Medications: including SGLT2 inhibitors, erythropoietin, testosterone
• Some tumours

Identified on FBC, either incidentally or in the context of other clinical features.

Consider investigation for genuinely persistently elevated Hct > 0.48 for women and >0.52 for men.

It is necessary to exclude spuriously elevated haematocrits due to sample delays, so please ensure a fresh uncuffed sample is obtained and delivered to the lab for processing with minimal delay.

Please exclude secondary precipitants as above.  This may include clinical assessment (with oxygen sats) and history, as above.  CXR will inform the assessment and further specific tests may be required.

If no obvious cause is identified following this assessment, please send EDTA blood sample via Blood Sciences for JAK2 mutation testing.  This should be requested by GPs with a view to informing the need for Haematology referral or otherwise.  It will need to arrive for testing in Aberdeen before Thursday (and avoiding public holidays) and as such is best sent on a Monday or Tuesday.

Note iron absorption is reduced in patients with genuinely raised RCM as a means of limiting Hct and reducing thrombotic risk.  Iron replacement therapy can lead to a rapid rise in haematocrit with potential for serious thrombotic complications.  Even if the MCV is low, iron should not be prescribed without caution and/or Haematology advice.

Note re ICE test availability (Nov 2023):

There has been a delay in the next set of tests to be included in the next addition for ICE. Any test not already on ICE can be requested using the 'Test not listed on ICE' functionality (users should have received training by the eHealth facilitators). NB: a paper request can be sent for infrequent or unusual requests.

EMERGENCY: thrombosis, Hct >0.6

NON-EMERGENCY: JAK2 positive

DO NOT REFER: obvious alternative cause particularly if JAK2 mutation screening negative.

As per primary care guidance.

Note that molecular testing may be best deferred to community testing in order to exclude reactive polycythaemia in the context of acute illness.

• Thrombosis.
• B symptoms (weight loss and/or drenching sweats) or splenomegaly.
• Contemplation of JAK2 testing despite alternative contributing factors, eg, if felt insufficient explanation for Hct level