Clinical Biochem: GP Tips

Clinical Biochemistry: GP Tips

With thanks to Prof Jonathan Kay (Consultant Chemical Pathologist, Oxford Radcliffe Hospitals), and the GPs of the

19 Beaumont Street

Journal Club

When should I worry about a low sodium?

► hyponatraemia

·        Increasingly common with more elderly people on diuretics and SSRIs, and many seem to be stable and asymptomatic.  SSRIs are a common cause.

·        Monitor. If patient is stable no need to be too concerned even if sodium is as low as 125.

·        Be aware of rare causes such as Addisons and SIADH

o       Addison’s; suggested by low Na and high K. Ask the lab to do a short synacthen test if clinically suspected

o       SIADH:  if suspect, check urine osomolality; if high this suggests SIADH

And potassium?

►Hypokalaemia

·        K levels go down in warm weather, harmless psueudohopkalaemia; no action needed

·        Worry if K level is < 3

o       Consider referral to exclude Conn’s syndrome if K<3 and no other cause (e.g. diuretics) and hypertensive. Conn’s syndrome is underdiagnosed.

o       In patients on diuretics, if hypokalaemic but K level is >3 there is no evidence this is harmful. If K level is < 3, switch to a k sparing diuretic. This is a better strategy than trying to treat with K supplements

►Hyperkalaemia

·        Usually an artefact due to delay in transportation

·        The rate of change is more important in renal failure, so monitor closely especially if k>5.5 and discuss with renal team if going up or >6

Is there a place for testing magnesium levels?

►Magnesium

o       Relatively cheap test, probably under requested

o       Magnesium deficiency causes symptoms similar to hypocalcaemia e.g. tingling, parasthesia, muscle weakness, leg cramps and tetany

o       Often seen with hypokalaemia and occurs with diuretics, chronic PPI use, GI loss, renal disease, chronic alcoholism and poorly controlled diabetes

o       If <0.7 is associated with QT prolongation

o       Consider testing therefore in, for example, an older person on diuretics with muscle weakness, leg cramps or parasthesia

What about statins and raised CK levels?

►Creatinine Kinase

·        Levels are higher in males and black people of African or Caribbean origin

·        If CK levels are >5 times normal on a statin, you should stop the statin. If less than this and asymptomatic, then no need to worry.

And slightly ‘dodgy’ LFTs?

►LFTS

·        Minor abnormalities very common with fatty liver disease

·        If transaminases < twice the upper limit of normal then further investigation probably not necessary for most patients (especially if clinical picture fits for steatosis)

·        Bilirubin

o       If <1.5 times upper limit of normal, no need to be concerned

o       If largely conjugated (>70%) and <3 times upper limit of normal, then it is likely to be Gilbert’s

o       Consider genetic testing for Gilbert’s. Can request on an EDTA sample requesting ‘genotype for Gilbert’s’ to get definitive diagnosis

Lipid testing: fasting or not, and how often?

►Lipids

·        For risk prediction use total cholesterol/HDL ratio which can be non-fasting (total cholesterol is not affected by eating)

·        With total cholesterol there is a within day variation of up to 25% and also the level tends to rise throughout the day as cholesterol is protein bound

·        Fasting test is required for TGs and LDL.

·        For monitoring, annual tests are enough

Useful sites for further information:

·        www.bettertesting.org.uk  excellent site to have as a ‘favourite’ with good tips on minor abnormalities

·        www.labtestsonline.org.uk     patient info site but very useful

Simon