The two pasts

For decades the standard equations estimating how well a person’s kidneys were working carried a race coefficient. Two patients with identical laboratory values received different estimates of kidney function depending on whether they were recorded as Black, and those estimates governed referrals, drug dosing, and places on transplant lists. In 2021 a joint task force of the National Kidney Foundation and the American Society of Nephrology recommended a new equation without the coefficient. That is a good correction, arrived at in the open, and it creates a problem that has nothing to do with medicine. Every stored estimate in every chart was computed under a rule the field has now repudiated. What is a health system supposed to do with them?

Recompute silently and the record becomes clean and false. It will assert that a patient’s kidney function in 2016 was a number no clinician ever saw, and the decisions taken that year, which were reasonable given what was on the screen, become unexplainable. Leave the old values alone and a discredited rule keeps speaking in the present tense. The dilemma looks unavoidable while the record has one timeline, and dissolves the moment it has two.

Keeping when a fact was true separate from when the record came to believe it turns one timeline into two, so a correction can show both the reading we now think was right and the reading everyone was working from, instead of replacing the second with the first and calling it accuracy. Databases have a plain name for this, bitemporal modeling, and it is old technology, not a research frontier. One axis holds when something was the case in the world. The other holds when the system came to hold that belief. With both, a record answers two questions that are usually collapsed into one and are not the same question at all. Given what we know now, what should the value have been? And what did we think the value was, on the day the decision was made?

The second question is the one that carries the moral weight, because it is the only one that can adjudicate a past action. An audit that asks why a claim was rejected last spring cannot be answered by today’s data, and neither can a person asking why they were told what they were told. A system that has thrown away its old beliefs can only offer the current answer, delivered with the same confidence it once gave the answer it has now erased. Our research notes on retroactive correction reach for the word gaslighting, and it is the right one. A number changing on a screen with no explanation, no trace of what it used to say, and no acknowledgment that anyone ever saw the old value teaches a person that their memory of their own record is unreliable.

What the second timeline buys is the ability to render a correction as a divergence rather than a substitution. The corrected trajectory draws as a solid line, the previously recorded one stays visible as a faded track beside it, and the point where they part carries the reason and the date. Nothing is hidden and nothing is pretended. The interface says, in effect, you remembered correctly, and here is what we now believe, and here is when we changed our mind and why. That is more work than an overwrite. It is also the only form of correction that leaves a person’s grip on their own history intact.

There is a general rule in here for anyone who keeps records about people, and it survives the technical details entirely. Never destroy a belief in order to improve it. Append the better one, keep the earlier one legible, and let the divergence between them be visible, because a record that can show its own changes of mind is not a weaker record. It is the only kind that can be trusted to change its mind again.