I was taught before asking for a test to always ask myself 2 questions.
What will I do if the result is negative?
What will I do if the result is positive?
The counsel was that if the 2 answers are the same: don’t do the test!
In neonatology we have for many years been trying to find accurate predictive tests for sepsis. In a child with possible clinical signs for sepsis, or a clinical situation that puts them at high risk, then an accurate test that could reduce antibiotic use, might be helpful. At present about 0.8% of cultures taken to rule out early onset sepsis, and 10% for late onset sepsis, are positive. So 99.2% and 90% of antibiotic courses do not help the infant, but select out resistant organisms. What we really need is a test which is rapidly positive, and specific for sepsis. A test which becomes positive the day after the antibiotics are already started, and which is sensitive but not specific is not much use to us.
It seems to be not much use in older children either. A publication in BMC Pediatrics reviewed the use of CRPs in neonates and older children in an acute care hospital. They showed that most of the tests did not have any impact on clinical management, and they cost a great deal of money.
They did not mention the blood loss, but in tiny preterm babies this can be a significant issue for any test we do. A blood test needing 0.6 mL of blood for example (such as the CRP requirement for the lab in our hospital) will often lead to the baby having substantially more than 0.6 mL taken, say 1 mL. And if repeated 10 times during a hospitalisation, lead to a substantial blood loss.
The Health Technology Assessment program in the UK is amazingly productive in many different areas of medicine. They have just published a systematic review of predictive tests for serious infections in children. Unfortunately for my purposes they excluded neonatal studies and patients under 1 month of age. But I am convinced they would find the same thing that they note in their discussion. ‘Both CRP and PCT offer similar diagnostic performance and are superior to WBCs. However, neither CRP nor PCT has sufficient diagnostic value to either confirm or exclude a serious infection, and thus their results must be interpreted in the light of clinical findings’.
Very often when CRP is requested, the answer to the 2 questions posed above will be ‘I will start antibiotics and wait until the cultures are negative before stopping them’. If that is the case, save the money and reduce the blood loss!