Background: Critical values are required to be phoned 24/7. Other abnormal results fall short of the thresholds used to define critical values and may only be required to be phoned during the day. Community-based requestors prefer not to be contacted unless a result is critical and contacting them requires substantial staff resource. It is common practice to add tests to requests to expedite diagnosis or clarify the significance of a particular result using algorithms. Methods: We devised algorithms for reflex addition of tests which allowed the differentiation of significantly abnormal results as either critical values or those that only require day phoning. Results: Algorithms identified 158 out of 309 tests as being critical (51%) over nine months. Reflex addition of serum bicarbonate identified 4% of serum glucose (24.9 -37.9 mmol/L) as critical. Use of estimated glomerular filtration rate by reflex addition of serum creatinine identified 68% of serum lithium (1.49 -1.99 mmol/L) as critical. Addition of serum potassium, calcium and magnesium identified 21% of serum digoxin (.2.49 nmol/L) as critical and addition of serum potassium and calcium to all samples with serum magnesium (,0.31 mmol/L) identified hypocalcaemia in all cases. The addition of serum creatinine and potassium as markers for rhabdomyolysis-induced acute renal failure did not help in the differentiation of serum creatine kinase . 4999 m/L. Conclusions: Use of reflex tests helped inform a phoning system based on the division of results into critical values and nonemergency abnormal values. This avoids disturbing requestors unnecessarily and conserves staff time at night.

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