Chapter 3 · Result interpretation considerations

Critical values in biochemistry: potassium 6.5 mmol/L, sodium 160 mmol/L, glucose 30 mmol/L, calcium 3.2 mmol/L. Reference intervals must be age- and sex-specific where relevant (creatinine, urate, ALP). eGFR must be reported with every creatinine. Liver function tests must be reported as a panel.

Chapter 3 · Practical example

TSH cascade — acting on suppressed results

A suppressed TSH requires appropriate reflex testing and a clinical comment — not just a numerical result with a flag.

▲ The situation

A TSH of 0.02 mU/L is reported on a patient attending the GP clinic with fatigue. The reference interval is 0.27 to 4.2 mU/L.

Reference: ISO 15189:2022 Clause 7.5; British Thyroid Association Guidelines

✓
Good practice

The laboratory initiates reflex free T4 testing per its documented algorithm for TSH below 0.1 mU/L. The free T4 comes back at 28.3 pmol/L (RR 12 to 22). The combined report: TSH 0.02 mU/L (below reference interval). Free T4 28.3 pmol/L (above reference interval) — initiated per reflex protocol. Findings consistent with overt hyperthyroidism. Urgent clinical review and thyroid antibody testing recommended.

✗
Poor practice

The TSH is released as 0.02 mU/L with an arrow indicating below normal. No free T4 is initiated. The GP, who sees many results daily, notes the low TSH and adds the patient to the chronic disease review list for a follow-up in six weeks. The patient presents to the emergency department two weeks later in thyrotoxic crisis.

● What this means for your practice

A suppressed TSH is a clinical signal requiring action from the laboratory — reflex testing, a specific interpretive comment, and an appropriate clinical recommendation. Releasing a suppressed TSH without reflex testing leaves the GP without the information needed to act urgently.